Is it too late? Help needed urgently please

Hi everyone, I'm after some advice.

I'm currently in Year 13. I did really well in my GCSE's and AS Levels, and I'm currently less than a month away from my first exam (it's on the 4th June). I have my firm and insuarance offers for University selected - and I need AAB to get in. Because of my AS results, in September I was targeted 3A*'s by my teachers.

I'm not going to go into deep details regarding my problems, but a brief summary is:
-Personal health issue arose last June, resulted in quite severe depression until January 2015. This meant the first 4 and a bit months of my A2 course were effectively missed. I was there in all the lessons, but I couldn't physically function and was there only in body. When I got home I couldn't bring myself to do work, I spent my entire time exhausted, in tears, or numb.
-The depression aspect was 'solved' in January following weeks of counselling - mainly thanks to convincing myself the health issue would be resolved after my exams when I had time to focus myself.
-It's been incredibly difficult to catch up on four and a half months worth of missed work, I'm pretty close, but still not there yet.
-Matters are even more difficult because the of sleeping issue which occured as a symptom of depression, meaning I cannot get to sleep before around 3am at the earliest (this has been the case for almost 12 months now, and I'm woken at 7am each day for school) means I am still in a constant state of exhaustion.

I physically don't think I can deal with this next month and a bit of exams. I'm nowhere near where I should be in terms of exam prep and revision, and there is a very real chance I won't get the grades that I need for Uni. If it wasn't for these extenuating circumstances, I genuinely think, as do my teachers, I would have been capable of straight A*'s, which makes my situation even more frustrating.

So my questions are as follows:

1. Is it too late for me to go into my 6th form tomorrow, and ask to withdraw from all my exams?
2. Is it possible for me to resit solely year 13 (and keep my AS results), next year?
3. Would my university that has given me a conditional offer be likely to allow me to resit 13 (it's quite a competitive course), if I inform them of my extenuating circumstances?

Thanks in advance for reading. I've tried my hardest to catch up from unfortunately circumstances, but I have physically reached breaking point.