I have a serious jealous problem w/ my boyfriend. Help?

(I have such a jealousy problem, it's horrible. I feel like i have a monster inside of me! I get so jealous when it comes to my boyfriend and idk why. We've been dating a year and a half and to my knowledge he's never cheated on me. Cheating has never even been an issue w/ either of us. But i just hate girls when it comes to him! On Facebook, i get so pissed off when a girl comments his status, or likes one of his pictutes. I automatically dont like the girl just for simple reasons like that. I'm 19 and this is just childish. And honestly, all of the girls that comment his sh*t or like his pics have N O T H I N G on me. And i'm not saying that just because most girls would say that about themselves, but i mean it. Ppl constantly tell me i'm a very pretty girl, so idk why i get so jealous when i know i look better than them. Ugh i just hate girls. And the thing i hate about my bf is that he's too friendly! Cant he just ignore their comments? Idc what they're talking about. Why does he need to respond to them? I just wish i wasnt like this. I was never like this at the beginning of our relationship and i guess i slowly progressed into this envious b*tch for whatever reason. I think it's because of past relationships? I've been heartbroken, blahblahblah. But instead of going off on my bf like i used to about girls talking to him, i just bury it down inside me. And as far as girls knowing about us dating, i'm sure they dont care! Because if they did, they wouldnt try to make an effort to talk to him by texting him or anything. I wish girls were like me. If a guy has a gf, i leave them be. I dont text him or write on his fb or anything. I just wish i knew how to deal w/ my jealousy because not only is it extremely immature, but i think it's slowly ruining me :(