Don’t go to sleep with ‘bad’ thoughts in your mind. Try to find a good memory of the past day and sleep happy.
Do not try to think for others. Do not make the mistake to think that you know how they think about you. If there is a problem, talk about it.
Be satisfied with what you have. Make a list of what goes wrong in your life and one with what makes you happy in your life. Read them and realize how lucky you are.
Do not continue to walk around with revenge feelings or dislike feelings. It is just unnecessary baggage. You feel happier when you focus on the future instead of the past.
Find solutions. Sometimes you are so involved in a problem that you do not see the solution anymore. Take a walk, read a book, get your mind off it and the solution will come to you.