Cheater

So I've been in a relationship for more than year and a half, and the guy is like really cheating on me, in my paranoid self I checked out his phone and found a lot of conversations with girls he knew from Tumblr, girls from Venezuela, Spain, Mexico... he have a lot of nudes, but with one girl specially, he text, a lot, and checking more and more I found out he is in a long distance relationship with her. Honestly I don't know what to do or even how to cope with it he is 24 the if he girl is 16, I mean she's a kid, but anyways she know about my existence and even make fun of our relationship. I'm beyond angry, and sad, and I want to break up with him, but I can't, besides I know he's gonna turn it on me for checking his phone... what should I do?