If you truly want to be happy in your life, learn to forgive others. There will be so many people around you who have hurt you terribly but make sure that you are not holding on to the grudges any longer.
It often gets easier said than done, but make sure that in order to gain happiness, you have no other choice but to master this art of forgiving others.
When someone has done something that was absolutely against your ethics, all you can do is to try that person with the right knowledge and thus, forgive that individual.
You must realize that when you let go and forgive others, you feel a lot more light and relaxed within yourself.
You will probably meet a lot of people in your life who would do wrong with you.
However, it is simply not possible to rectify each and everyone but make sure that you do not have any sort of pity feelings for anyone in your heart.
Focus on your own goals and be determined to live your own life. Letting go is a trick to make sure that you are not banging your head thinking of something that has already happened, and you have no control on it.
Try to follow your passion instead, and do things that make you happy.