I know exactly how you feel- I'm in my first year in a life sciences PhD program which I'm starting 10 years after graduating from my science undergrad. In-between, I went to law school and was practicing law. I am planning to take my comps at the end of the summer (for us its a 3 day written and 1 day oral). I just had my first committee meeting in which I had to present my background and research to date as well as my plans for my future research project. It was terrifying because I really felt that I did not have adequate background knowledge in my subject area. However, what was helpful was that in the months leading up to my meeting, I set meetings with my advisor to discuss very big picture what I was thinking of doing for my PhD project. After I got the big picture direction, I went back and did some research and after discussing with my advisor what would work, we broadly discussed experiments I could do. I then went home and made a much more detailed outline which I've reviewed with her a few times until I felt I got the bearings of what would be important in my area to focus on to start filling in knowledge gaps. Going through this process in stages big picture to more narrow meeting with my advisor and discussing things as well as spending the time to really write things out (or in my case diagram as well- I like pictures) made me feel more confident. I'm slowly learning that the knowledge gaps will be there (and are expected to be there), you just have to keep moving forward in identifying them and filling them in as you go. I also have friends in the science field and before my presentation to my committee on my research, I made sure I practiced presenting the material to them which helped me tremendously.