How do I stop thinking about my girlfriend who cheated on me?

It can help if you teach yourself to not associate things you do with her, as thats something most of us are guilty of after a break up (I remember when we went here, did this and that etc). If you think of her when you listen to certain songs/watch certain movies, then over play/watch them until you no longer think of her. Also, as hard as it may be, throw away things that she gave you if you feel comfortable in doing so. Or again over use them until they have no connection to her. Remember that you're strong enough to get through this :)

Getting her out of your mind can be very difficult and letting go can be ever harder. Just know that it wasn't meant to be and her cheating proved that. You may not be able to stop thinking about her for a while; you truly cared about her and letting go of someone you care about is hard, but if it was meant to be it would have been. It didn't work out which means that there is someone else out there who is better for you.