Cheater Cheater Cheater Cheater Cheater! What do i do?

Me and my boyfriend have been dating for a year and he has cheated on me twice. The first time i found out and it hurt like hell. At that time we had been dating for three months. It was emotional but i never broke up with him for it We stayed together and then i had to literally track her down and threaten to kick her as* before she would stop talking to him and i chewed his as* out too about it. My boyfriend is a really big flirt and I'm the first girl he has stayed with for more than a few weeks at a time. Well after he cheated on me then i got really insecure and i did make him stop talking to her. Then a month ago me and him starting fighting really bad about me smoking so we broke up. Then when we got back together 3 days later i find out that he had cheated on me almost a week before we broke up. So i broke up with him again but i found out that he had been under the influence of ganja when it happened. Either way i still get paranoid because he still talked to her for a few weeks after it happned. Well i started to fall for a guy friend of mine and now we talk all the time and so my bf made me stop talking to him but i still tak to him. I love y boyfrind. but i cant trust him like i used too and fnd myelf so paraniod. but he always tells me that he messed up big timeand he couldnt live without me and he didnt know what he was doing. i dnt know wat to do. What should i do?