Why is it so hard to let this jerk go?!?!?

I'm 17 & have been on & offw/my bf (20) for 4 yrs. He used to be nice but all he's done since finishing high school is sit on his *** playing online videogames & flirt with a bunch of tramps. Basically he's become a jerk. He calls myabe once every 2-3 weeks and will randomly stop our conversation. He tells me he "loves" me all the time, but his actions say that's bs. I've dated other guys,but he was my first bf & the only guy i've ever loved. I just wanna stop being in so much pain! That's all i feel with him anymore. He doesn't have a job & doesn't plan on college. So....why can't i get over him??? I'm just going to leave him & not even bother telling him it's over. He can't even get so many girls in real life so what's with all the online tramps?? He's even told them he "loves" them! Wtf! He makes me feel so bad about myself, so why is it so hard to let go?? I hate that he's changed so much. It's like he's only there for me when it's covenient for HIM