Just stop thinking of all the happy moments and train yourself to think of all the bad moments with her/him enough to make you not even want to think about them no more, and also just keep yourself busy and take some time away from dating till your ready, don't jump right into dating someone else ether as your head is not clear enough, have fun with your friends and just do what you enjoy without your ex to get used to her/him not being around, remember they are an ex for a reason and the things they did should not be forgotten because then it will help you learn from them and figure out ways to better your future relationships, but for now just chill and have some breathing time.
Put yourself out there. Go have fun being single with your friends and hanging out with the people who you have fun with. And then when you meet a MUCH better girl along the way (because it usually happens when you least expect it) you'll almost forget the last girl exists. Sitting around at home is the last thing I'd suggest, because I know when I'm bored my mind likes to wander. You need to keep yourself busy.
Live happy, be successful, don't dwell on the negative, be happy you found out sooner than later that the person you were with is a scumbag with no morals, find new people to spend time with, improve yourself, find new hobbies, help others in need and just keep busy. Good luck brother