You're young and inexperienced in relationships which is why I will cut you some slack. Basically, you're in a bad relationship where you love your boyfriend, but he does not love you in the same way. He cheats because he can and you keep forgiving him. 

You need to end this relationship for your sanity and emotional well being. There are plenty of boys/men who do NOT cheat on their women. Trust me, it's much easier to have a relationship in which you can trust your partner 100% to not lie and cheat.