Casual_Bongos

I also had a gap between my undergrad and grad. When I applied for grad school I had been out of school for 10 years so I had very little hope that any of my professors would remember me. I did have a letter from one professor that I had gotten before I graduated but when I tried to get in touch with him I found out he had retired. A second professor who had also written one for me was now working overseas so she was pretty much out as well.
What I ended up doing was sending in letters from three people I had worked with over the last 10 years (I was working in the same field I was planning to go to grad school for). I know people say don't do that but honestly the letters from my former bosses were a much more accurate example of who I was now as a 30 year old grad candidate then the undergrad one that talked about 20 year old me. I sat down with the professionals who were writing my letters and made sure they touched on how quickly I learned new skills, whether or not I helped to develop a new process or streamline something, and my contributions to our field.

In the end the grad school accepted me and I never heard a peep about not having a professor writing a letter for me.

So in your case I'd say get the TA professor if you can and then hit up someone you're working with at Planned Parenthood (but sit down with them and make sure they address your ability to learn and contribute to the field rather than just saying you're a "good worker"). If possible you may also want to try to find some work more in the field of Public Health. Perhaps a part time job or even full time if you can manage it. That'll help you net a relevant 3rd letter and show your future grad schools that you're serious about making a career change. If you can't, you're going to have to figure out a way to make your current job fit with Public Health so you can use a letter from that company. I don't know what you do at your current job but if there's some kind of health and safety department or concerns I'd start buddying up with those in charge of that and helping them out. Then you might be able to use that person for your 3rd letter talking about your concern for the health and safety of your fellow workers and what a great help you've been with making sure everyone is safe in the workplace.

I always encourage everyone to try and apply, even if the university you're looking at says they won't consider you because you absolutely never know.
I had a 2.7 undergrad GPA and was on academic probation twice while I was an undergrad. Back when I was an arrogant 20 year old I was the poster child for "C's get degrees!"
About a year and a half ago, I applied to a university that flat out said "if you're under a 3.0 we won't even look at you" and brags the average accepted student has a 3.8 GPA. I was very aware that my 2.7 was going to be a problem so did I everything I could to address it before sending my application.
1. I studied like crazy to make sure I got a decent GRE score, which I could then use to show that I'd mended my slacker ways. Ended up with 168 verbal and 5.5 analytical writing. Quant was a garbage 144 but submitted it anyway because I wasn't applying for a math based program so who cared. I was applying to a research heavy university and I suspected the verbal and writing was what they truly cared about.
2. Applied for an "off season" start date. I knew most people apply for Fall start dates, which meant I was going to be competing with a lot more people if I went with Fall. So instead I opted to go for the Spring start date to increase my chances.
3. Work experience. This is, honestly, what I think saved me. I had 10 years of work experience in a related field and I made sure they knew all about the projects I worked on and the achievements I'd gotten from my companies and clients. In the SOP I essentially said "Yes, my undergrad GPA was garbage but look at all the amazing things I've done since then. I have grown up, matured, and proven that I have learned my lesson and can be successful in this field."
4. I didn't ask for an assistant-ship or financial help from the department. Again, to increase my chances I didn't want to look like I was competing with other applicants over the limited financial opportunities and I didn't want them to consider me as a "burden" on the department in any way. (This didn't mean I couldn't ask for it or get help later. The department actually offered me a TA job my second semester despite me telling them in my application that I didn't need it so I only ended up paying tuition for my first semester.)

With that plan in place I basically YOLO applied to that one school and two weeks later they sent me an offer of admission. Like I said, I really didn't think I was going to be accepted because literally everything the school and department said indicated that someone like me was never going to be considered and it is a top 10 school. 
So, you never know. Even if you have a low GPA and are nervous, do your best to show you've matured since then and give it a shot!

I always encourage everyone to try and apply, even if the university you're looking at says they won't consider you because you absolutely never know.
I had a 2.7 undergrad GPA and was on academic probation twice while I was an undergrad. Back when I was an arrogant 20 year old I was the poster child for "C's get degrees!"
About a year and a half ago, I applied to a university that flat out said "if you're under a 3.0 we won't even look at you" and brags the average accepted student has a 3.8 GPA. I was very aware that my 2.7 was going to be a problem so did I everything I could to address it before sending my application.
1. I studied like crazy to make sure I got a decent GRE score, which I could then use to show that I'd mended my slacker ways. Ended up with 168 verbal and 5.5 analytical writing. Quant was a garbage 144 but submitted it anyway because I wasn't applying for a math based program so who cared. I was applying to a research heavy university and I suspected the verbal and writing was what they truly cared about.
2. Applied for an "off season" start date. I knew most people apply for Fall start dates, which meant I was going to be competing with a lot more people if I went with Fall. So instead I opted to go for the Spring start date to increase my chances.
3. Work experience. This is, honestly, what I think saved me. I had 10 years of work experience in a related field and I made sure they knew all about the projects I worked on and the achievements I'd gotten from my companies and clients. In the SOP I essentially said "Yes, my undergrad GPA was garbage but look at all the amazing things I've done since then. I have grown up, matured, and proven that I have learned my lesson and can be successful in this field."
4. I didn't ask for an assistant-ship or financial help from the department. Again, to increase my chances I didn't want to look like I was competing with other applicants over the limited financial opportunities and I didn't want them to consider me as a "burden" on the department in any way. (This didn't mean I couldn't ask for it or get help later. The department actually offered me a TA job my second semester despite me telling them in my application that I didn't need it so I only ended up paying tuition for my first semester.)

With that plan in place I basically YOLO applied to that one school and two weeks later they sent me an offer of admission. Like I said, I really didn't think I was going to be accepted because literally everything the school and department said indicated that someone like me was never going to be considered and it is a top 10 school. 
So, you never know. Even if you have a low GPA and are nervous, do your best to show you've matured since then and give it a shot!

I agree with Kita. Do not criticize your former school or teachers. The people reading your SOP are either going to be teachers themselves or on the side of teachers and schools. they're not going to look kindly at student who seems to be anti-teacher or comes off as "knowing better" than their former college and teachers. They don't know you yet and you don't want them wondering whether or not you'll be badmouthing them when you leave.
Instead just acknowledge your low GPA and then go into what you've accomplished since then to prove to them that you've learned your lesson or corrected whatever the cause of the low GPA was. In your case you may want to paint it as "(Former University) offered a variety of outside projects that I became interested in. Unfortunately I failed to maintain a proper classroom - project balance and my grades slipped. I realize now that I needed better time management skills and have worked hard to correct that. In addition, my involvement with XYZ projects helped expand my knowledge by ...." They'll appreciate you being up front and taking responsibility for your low GPA and should hopefully realize you will work to do better at their school.
Good luck!
 

I also had a gap between my undergrad and grad. When I applied for grad school I had been out of school for 10 years so I had very little hope that any of my professors would remember me. I did have a letter from one professor that I had gotten before I graduated but when I tried to get in touch with him I found out he had retired. A second professor who had also written one for me was now working overseas so she was pretty much out as well.
What I ended up doing was sending in letters from three people I had worked with over the last 10 years (I was working in the same field I was planning to go to grad school for). I know people say don't do that but honestly the letters from my former bosses were a much more accurate example of who I was now as a 30 year old grad candidate then the undergrad one that talked about 20 year old me. I sat down with the professionals who were writing my letters and made sure they touched on how quickly I learned new skills, whether or not I helped to develop a new process or streamline something, and my contributions to our field.

In the end the grad school accepted me and I never heard a peep about not having a professor writing a letter for me.

So in your case I'd say get the TA professor if you can and then hit up someone you're working with at Planned Parenthood (but sit down with them and make sure they address your ability to learn and contribute to the field rather than just saying you're a "good worker"). If possible you may also want to try to find some work more in the field of Public Health. Perhaps a part time job or even full time if you can manage it. That'll help you net a relevant 3rd letter and show your future grad schools that you're serious about making a career change. If you can't, you're going to have to figure out a way to make your current job fit with Public Health so you can use a letter from that company. I don't know what you do at your current job but if there's some kind of health and safety department or concerns I'd start buddying up with those in charge of that and helping them out. Then you might be able to use that person for your 3rd letter talking about your concern for the health and safety of your fellow workers and what a great help you've been with making sure everyone is safe in the workplace.

I agree with Kita. Do not criticize your former school or teachers. The people reading your SOP are either going to be teachers themselves or on the side of teachers and schools. they're not going to look kindly at student who seems to be anti-teacher or comes off as "knowing better" than their former college and teachers. They don't know you yet and you don't want them wondering whether or not you'll be badmouthing them when you leave.
Instead just acknowledge your low GPA and then go into what you've accomplished since then to prove to them that you've learned your lesson or corrected whatever the cause of the low GPA was. In your case you may want to paint it as "(Former University) offered a variety of outside projects that I became interested in. Unfortunately I failed to maintain a proper classroom - project balance and my grades slipped. I realize now that I needed better time management skills and have worked hard to correct that. In addition, my involvement with XYZ projects helped expand my knowledge by ...." They'll appreciate you being up front and taking responsibility for your low GPA and should hopefully realize you will work to do better at their school.
Good luck!