Firstly let go of the people that ain't good for your mental health. Even if you can't do it due to various reason, stay away from them as much as possible.
Mind your own business. Do your job. Be responsible. When you are leisurely available, read a book, learn a new skill you wanted to try, go for a walk and things like that. But don't get involved in scenarios that drains your energy.
Seek peace over anything. Practice gratitude. Focus on improving your life. Track your bad habits and work on them one at a time. Spend time all alone and get to know yourself. Rely on nobody. Don't get emotionally attached with anyone. Fulfill your duty and responsibilities and that's it. Don't go beyond that.
Expect nothing from others. Be your own best friend and treat yourself right. With dignity, self-respect and self-care.