How do I get my BF [21 M] to stop cheating me [20 F]?

We have been dating for 2 years now. The cheating first started about 2 months into the relationship. When I first caught him he apologized and said it's probably best we end things. I ended up texting him after a few days and we got back together. But the cheating didn't stop.

I've tried many things. We share a facebook account. I randomly check his phone. All of that. But he keeps cheating. He said he loves me and I know it's true. I just want him to quit cheating. How do I get him to stop?

tl;dr: bf keeps cheating. I want him to stop.