How to cope with life changes and burnout after PhD thesis submission?
The first thing to do is to get rid of the 90 hour weeks, and resolve never to get back into that trap. There is a lot of experience suggesting that the maximum weekly output for intellectual jobs is produced at around 40 hours a week. Going beyond that reduces actual production, because of more mistakes and less creative thought. Your mileage may vary, but it would be very unusual for 90 hours to be optimal.

You need to accept that sleep, exercise, relaxation, and social life are all essential to achieving maximum productivity when working, and to general mental and physical health. If you feel intense guilt when not working, and cannot change that by yourself, you may need some sort of counseling to help you get a healthier attitude.

The mental issues you describe could just be because you got into a bad habit of working too long and relaxing too little. On the other hand, there could be an underlying problem that needs a professional. I am not qualified to evaluate that, and agree with the suggestion in comments that you should consider counseling.