Discuss with someone sensible and fair-minded what went wrong. Get it clear in your mind the areas that you could do something about and improve in yourself rather than dwell on any misdeeds on the part of your ex.
Then get out and involve yourself in social activities. If you can find voluntary work that you really enjoy, the act of helping others will take you out of yourself. It's sitting and going over and over what someone has done to you, feeling resentful, hard done by and bitter is the worst possible thing.
You can't get over it that way. Change something about yourself to feel you're making a fresh start: your clothes, your regular restaurant, even your route to work, whatever. 'I'm starting anew and I'm going to enjoy life' needs to be your attitude. Hard at first, but involving yourself in activities will work.