There is lots of advice and support from people going through this on the 'struggling with separation' threads. I found the Runaway Husbands book helped a lot, and yours sounds like a runaway situation. Other things that helped me were a good counsellor, long conversations with supportive family (I found it very hard to tell wider people, work colleagues etc), and the classic self care recommendations that everyone else is suggesting above - the basics of diet, sleep, exercise.
In the early stages I made a conscious effort to make beneficial choices rather than destructive ones. Not very strictly, e.g. I used wine a lot to relax but reined it in when I could see my consumption increase.
If you are struggling to sleep there are really good sleep visualisations on Spotify. I kept a notebook by the bed and if I couldn't sleep I would write things out and try to empty my head. Then I would listen to a visualisation recording to try and soothe back to sleep. As others have said, lots of exercise helps with sleep too. I have dogs so needed to walk them every day and getting outside and keeping a routine helps.
The self care stuff shouldn't be punitive or another thing to beat yourself up about if you don't e.g. cook a healthy meal from scratch every night. I was exhausted for quite a long time just dealing with all the practical and emotional stuff that separation brings. I have watched an insane amount of TV in the past year, but I think it's just been about allowing my brain to switch off for a while. I felt better after I read a study that said low-effort activities like watching TV are exactly what is needed to recover from or prevent burnout!
For me, self care is a combination of doing what makes you feel better at that moment but also understanding what will really make you feel better in the long term e.g. a tub of ice cream or bottle of wine might be genuinely what you need now and again but if it's every night it's only going to make you feel worse and make it harder.
The first stages are really tough though. Just getting through the days is an achievement. Don't set your expectations of yourself too high. Good luck.