hello and welcome to the forums.
having mental health issues and completing school sounds very difficult. And I see from your post that it also makes going back to the school, to resume, is much harder when you are behind. So I really feel for you in this instance.
with that said... I know from experience there are many ways to get into Uni and into your preferred course. For example I did not get into Uni and went to TAFE and doing the course there enabled me to get in. On top of that, the course I completed also gave be credits at Uni and did not have to do some subjects. In speaking about this with my psychologist... we all have our own journey and for me to go to TAFE was OK. Fwiw, my grades in high school were average for other reasons.
Perhaps you might be able to chat with someone in school that performs a counselling type role about your options, where things are, what your dreams are etc. and hopefully be able to find one to move forward from how you are feeling at the moment.
There are more stories I could give.
In this moment it seems your dream is unreachable. If nothing else, know there are other ways to get there and beyond.