It just won't leave me alone

My ex and I where together for about 4 months, we had amazing times together and we had our really bad points. When he broke up with me I was fine, but about a week after I realized that I was in love with him. It's been6 months and I still miss him so much. It's gotten to the point when if I see him out with someone else I cry out of sadness and anger. However I have realized that... he is such a jerk. I don't need a guy like that, he knows how I feel, and so he plays mind games with me. I've decided I need to let him go no matter how much it kills me. No matter how much it hurts.