Hey. Sorry you are in this situation. My only advice would be to NOT send the email you shared with us. You can thank them for the opportunity and as another poster said explain that due to medical reasons or whatever you need to resign (if you truly do).
But I would not go into detail and definitely not take Responsibility for not doing a good job, having wasted their time, blown it, or lost your credibility, etc. Let them decide what you did or didn't do- don't give them ammunition that could later get used against you. You may need them as a reference, or businesses talk so if u later look for a job in the same field it could Come back to bite you in the arse.

Do you truly need to/want to resign? If you need to get out of your lie, could you somehow say that it turns out cvid was a misdiagnosis, and that it turns out you actually have a mood disorder, for which you could get a real medical release form from a pdoc? Idk, I am not generally in favor of sharing a MI dx with a workplace but in this case it might save you. It's just one idea. I'm Really sorry you have had to deal with this pattern of career success followed by self sabatoge. Therapy might help you to resolve this. Hang in there. Good luck.