My advice here would be a two pronged approach. First, I think you need to have a face to face discussion with your manager to explain the situation. The conversation may even be off the record, but a conversation should happen in my opinion nonetheless. Second, you should follow up with a professional, succinct letter formally announcing your resignation from the position due to personal reasons.

Then in your letter simply state that effective immediately (or two weeks, depending on how your conversation goes) you resign from your position due to personal reasons beyond your control. But by no means send an email where you claim to be "dead wrong" and that you have "blown it".

Edit

I don't think that quitting is career suicide unless you are truly out to self-sabotage and burn as many bridges on the way out as possible. You may need to get your personal affairs in order first before you begin working on your career. Obviously gainful employment will get you much easier access to healthcare and getting the help that you need. But, if you cannot work due to your condition it may be time to explore other options.