Hi there,
I also battle with severe depression and suffered through Year 12. I stuck with Year 13, had a mental breakdown and was physically ill during the Christmas period of 2014 and dropped out of college in February 2015. You've still got pretty decent predicted grades despite your troubles! I was averaging CCD in Computing, Maths and Chemistry respectively when I dropped out.
I'm now near the end of my second A2 year and I'm predicted AAB. I'm more so aiming for ABB, which is what I need for York, so I guess I'm in a good place.
Like you, I just didn't take anything in for the whole 2 years I was in college (before I dropped out). Depression can do that.
Now I don't know what your sixth form/college is like, but the first major step for me was letting the people around me know I was ill. I was blessed with some wonderfully supportive tutors (bar one, my Chem teacher, he was an arse). Once I told them about how I was (at the beginning of my first A2 year) I began receiving support. Sometimes it wasn't entirely obvious. Tutors being more proactive in asking if I needed help, knowing not to ask on me in lesson if I told them I was having a rough patch, stuff like that. Unfortunately for me, the medical side of things failed to help. I was left waiting for about 7 months for any kind of therapy and was constantly yo-yoing from medication to medication, which made me physically sick 90% of the time, hence why I left.
Coming back to college for a third year was scary, but it was the best decision I made. I'm not saying it's your course of action, though. You're in Year 12 and can still improve in your second year. In a way, my main motivator for this year is the fact that A-Levels will be over soon for me and I'll be at university, like I should have been 12 months ago. In Year 12 that seems like a life time away, I know.
I can't give a magic word to motivate you, but I can say a few things that might help make things easier. One is that you'd be surprised how many people struggle during A-Level. It's incredibly common and the people who find A-Levels easy are few and far between. I know on TSR it can sometimes seem otherwise, but don't let it fool you.
Another thing that can help is to not beat yourself up and to know your abilities and your limits. It's only recently that I've realised that doing my best does not mean working so hard that I make myself ill. Try and make the most of the hours you're in lessons and let your tutors know how you're feeling. If tutorial sessions are offered, try to attend some (that way you've at least done extra work for the day, just by attending) and don't be afraid to ask for help if you find that you just can't grasp something. Again, I've been lucky to have some lovely tutors, but I have faith that 99% of tutors will empathise with you and will be willing to help you. If you have a personal tutor/form tutor let them know and see if there are any counselors on site that maybe you could see. From what I've been told, things can be kept confidential and your parents/guardians are only informed if you're in any danger, but if you're just asking for advice then everything should be kept under lock and key.
Just know that you can do this and being ill does not define you. I spent many nights sobbing, thinking that I was genuinely stupid and that being ill was no excuse. Like I said before, you still have good predicted grades and you should be proud of them, they show that you're capable, even while struggling. Feel free to ask me any questions.