So outdone and I don't know how to feel

I though me and my "boyfriend" were working things out pretty well until I just saw him literally 5 min ago all over this girl in her Snapchat. I just sat in the bathroom and cried my eyes out because I've given him my all and been nothing but supportive of him. My mom was even beginning to like him some too. I'm just at a loss for words right now because I always go back to him thinking that he's changed and he's really shown me what type of person he is. I just don't know what else to do. I feel like giving up on relationships in general because I'm always the one left hurt. Have any of y'all ever been through anything like this?