ughhhh

I don't think this is really the place for this but here it goes anyway. I'm a first year in a PhD program and I need to get of grad school. It's taken an extreme toll on my mental health, exacerbating underlying issues that I won't get into here. Nevertheless, I'm woken up each morning by intrusive, anxious thoughts, I'm constantly exhausted, I can't concentrate. The academic life is not for me and I need to leave, ASAP. I spoke with my departmental advisor earlier in the year (different than my lab PI) and told her that I would most likely not be continuing in the program. However, I have a teaching assistantship, so I told her that I would honor that obligation by finishing out the semester. She essentially advised me to keep my feelings a secret from my lab and PI until the end of the semester. Fast forward to today- I can't maintain this charade. It's been a month and change and I don't think I can last another two months. By mid-November, I'm expected to begin a project (with another student in the lab) and if I refuse, they'll know that something is up regardless of my best attempts at secrecy, and if I do begin the project, I don't think it's fair to just dump all the unfinished work on my colleague at the end of the semester. (Not to mention the awful feeling of seeing my advisor multiple times a week, smiling in his face like I'm the perfect, motivated grad student.) The sole thing that is preventing me from leaving right now is my TAship. I don't know how I can just up and leave mid-semester when I am responsible for 80+ students. I feel like it would be wrong and very unprofessional to do so. Yet, I literally don't know how I can finish the semester without suffering a mental breakdown. I feel guilty, extremely depressed, and trapped. I don't know what to do or how to make an exit. What would your advice be for someone in my position?