Really need to avoid this.
Get outside and get involved in a bunch of activities, even if not feeling it. -learn a language, sportsEtc. Find a local library/student friendlycafe and treat it as your own office/base for the day anything to be not slouching around the house.
Maybe start researching and learning about an industry you are interested in. I would say do a bit of studying/prep for going back to college but that can be a double edged sword and if sets you off then forget it.
Force yourself to be social at least once or twice a week, try and find a crowd that is upbeat and gets you laughing. Stay the hell away form anyone negative and whiny
Good that you’re getting vit D supp but also make sure get 20+ mins outdoor daylight as well, sun on face etc
As you rightly said junk food not good either -get in tons of greens and colourful veg, lean protein good fats. Drink plenty of plain water between meals. Avoid sugary crap.
Fermented foods and improving gut health might be of help also…