Why cant i let go?

I met this guy in july .he seemed really nice. In august we started spending alot of time together. I asked him if he were seeing anyone or having sex with anyone else. He said that he goes on dates and that he had had sex with some one else twice since meeting me. I told him i would step back and let him do his thing and he said he didnt want me to step back. he asked me if i wanted him to stop seeing other people and i said it was up to him . he said that he would stop because he really likes me. so time went on and i started to catch feelings. i would tell him how i felt and he was ok with that. he said he was not ready to commit to me. he was not ready for a relationship.But he treated me as if i were his girl.one night after he went out i came over and while we were having sex he asked me to have his son. I told him lets not talk about this now. he said what kind of realtionship do we have if im not willing to have his son.the next morning he acted as if he never said those things. in october i asked him if he was my man or not . he said he made it clear that he didnt want a relationship.we argueing for 2 weeks straight everyday sending emails and on the phone. he would never try to see things my way or listen to me.out of the blue october 31st he called me and said if we have sex are you gonna start tripping again. i told him no. we had sex and he didnt call or email me after that. when i would call him he would be short with me. this past friday i called him and asked him to come over he did and i asked him why he did that to me . he said he knew he made a mistake and that he knows i want a relationship. i told him he should have thought about it before he had sex with me.for the past month i have been hurting because i really cared about him. i dont know how to let go.