What do I do?

Yesterday, I found out that over Christmas break, my boyfriend Snapchatted and texted a girl I go to college with. He was asking her boob size and asked if he could see them. Then, he wanted to send a dick pic and was telling her that he was horny. Then, when I found her name on his phone back when, he denied it and I believed him because there were no messages. He lied about it everytime I asked him. We've been together a year and a half. What do I do?