1) First, regulate your interactions with the environment i.e. limit exposure to everything/everybody which could trigger you, put you in a negative frame of mind etc. A good example is to do a "digital detox" in addition to limiting socialization with negative people. Clean up your room, change some decor etc. The idea is to break those cues in the environment which have been detrimental and replace them with something positive.
2) Next start with your Diet; stop all junk food and eat proper home cooked healthy food. Stop eating before you feel full i.e. satisfied but not completely full.
3) Regulate your Circadian Rhythm by following the six steps given by Dr. Satchin Panda in this talk - https://www.youtube.com/watch?v=fciGNBN0nKM and this talk https://www.youtube.com/watch?v=qOXQgyjRm0I Note especially Intermittent Fasting, Sunlight Exposure in the Morning and Exercise in the Evening.
4) The "Gut" (aka The Second Brain) is now known to play a vital role in our Health both Physical/Mental. Massage all the organs in the gut area everyday by doing two special exercises from Hatha Yoga; "Uddiyana Bandha" (https://en.wikipedia.org/wiki/Bandha_(yoga)#Uddiyana_bandha) and "Nauli Kriya" (https://en.wikipedia.org/wiki/Nauli). Start slow and gentle with these exercises. See Youtube for some demo videos.
5) Your Breath links your Body and Mind (i.e. Psyche) together and hence you need to regulate your breathing. Do the following couple of simple breathing exercises (i.e. "Pranayama" from Hatha Yoga); 4.1) Normal inhalation followed by fast and forceful exhalation with abdomen pulled in, all through the nose; do ten reps 4.2) Do "Nadi Shodana" i.e inhale gently through the left nostril by closing the right nostril and exhale gently through the right nostril by closing the left nostril. Repeat the same but switching sides. This constitutes one set; do ten such sets.
6) Tackle Sleep as if it were a paying job i.e. with discipline and dedication. Satchin Panda's above video already mentions how but to drive the point home, listen to your body and sleep/nap when you feel sleepy/drowsy. Everything else is dispensable but not Sleep.
7) You don't have to go crazy with exercise but should at least be able to take a walk everyday. If you can't go outside walk within your home or building. For better results use a weight vest and/or ankle weights. Since we all spend so much time sitting and staring at a screen do 6.1) Special/extra exercises for Waist/Pelvis/Hamstring/Knee loosening 6.2) Massage your scalp/face/head to balance circulatory function. Everyday exercises should be light which energizes you but not tire you out.
8) The above together should have put your Body/Mind in a equilibrium state where you feel alert/calm and energetic. Now you work on your focus/concentration by reading/studying something from a book (no phone/music/videos and no other distractions). Pick anything you find interesting (trivial/simple/complex/whatever) be it fiction/non-fiction; you are training your mind to focus on one thing for an extended period of time (at least 1 hr). If you don't like to read then do some other activity (eg. crossword/puzzles/whatever) but again fully focused for at least an hour. The idea is to have the mind fully occupied (aka mindfulness) with no other thoughts/distractions.
Persist with the above discipline and you will definitely see some real benefits.