Why would he do this to me?

My boyfriends ex girlfriend just moved to our school and he hates her. I think she is nice but again she hates me. So we went out to eat and my boyfriend got mad because she was there with her boyfriend and just when I was about to sit down he snatched the seat from under me and comes running to me to help me up then drops me and helps me up again. We started eating and she.comes over and tells him hi but when I tell her hi she tells me that she saw my nasty fall and it was almost like my nasty face. So I get up and start yelling at her and so does my boyfriend and so her boyfriend comes and starts yelling at him we ended up getting kicked out the resturant. Today its my birthday and i was having a great time and with him until he randomly leaves and i see his ex at my party and the next thing i know he comes out of nowhere running and trips and pushes me into my cake. While i was in the bathroom trying to get cake out my hair he came in and i asked him why he did that and he said he didnt know that there was something about her that makes him go crazy and that he was so over her and that i was the only one he loved and that he made a desicion to be with me instead of her. I dont know what to do