How to cope with life changes and burnout after PhD thesis submission? The last year has been ridiculously difficult. I can easily say it has been 90h weeks 12 months in a row, with the hours taken of rest being vastly overshadowed by intense guilt (over not using that time to work) and inadequacy. I produced a lot of research, published 5 new papers, and produced a thesis which is now submitted for evaluation. On the day I submitted it (about a month ago), I couldn't even read a single sentence and maintain focus - that's how severe my burnout was. My sense of not being good enough permeated every aspect of my being.

I'm finding myself now getting better, slowly and surely. But the brain fog is ridiculously severe. I don't think I have lost intelligence, but the lack of care I feel for my current position as a research assistant eclipses all output I know I'm otherwise capable of producing (or used to be, anyway). On top of that, I feel more tired than usual. I easily need 1-2 more sleep in a day. Normally, I'd be exceptionally motivated to work out, and now I really could not care less. I adopted this "You don't have to do anything you don't want to do." attitude, in hopes to be kinder and gentler to myself as opposed to the whoo-tshh (whipping sounds) dictator I had employed to stay on track in my madness PhD.

I cannot fathom a job in this field anymore, I cannot stand the endless thinking up of made-up projects to pursue research grants. I want out of academia, and this both excites and terrifies me. I feel largely inadequate as a computer scientist joining industry post-PhD because I just spent 3 years of my life not getting industry experience.

I feel lost. Like there's a daily existential crisis. I don't know what to do with my life, and what's worse, I don't even know if I'll recover from this. I mean, what even is this? PTSD? Burnout? Needing a break after excruciating intense labour and emotional distress?

How can I deal with the life changes and the drop in motivation after having submitted my PhD? How do I deal with the remaining burnout?
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.