How much do you have to put up with a person before finally leaving them?

I am a very young mother and I have a beautiful little girl with my fiance. Our relationship seems to be getting worse, he treats me so badly sometimes I just want to crawl under a rock. Then at other times he treats me so well and I honestly love him. I really have no place to go except my mothers. I work and he atteneds school. but he acts like he's bringing in all the money, his family thinks i ruined his life and he never defends me when it comes to them, we fight so much I don't know what to do I feel so stupid for staying with him. he tells me he will leave and break up with me just because I get upset or need to vent then when he needs to relieve stress it I have to listen to him. I know the best thing to do is leave but I don't know why I can't can someone please help me?