Lost on what to do?

I don't normally do this.. but I just need advice from other people. This is gonna be a little long so I apologize for that. Anyways I've been in a relationship for close to a year, and it's been wonderful.. for the most part. He's my first love, and I gave him myself. The past three months or so, he's been being a major dick. He treats me like a piece of shit. He wasnt a dick until he started working where he works. This one girl he works with I dislike because she tries to get with every guy. We almost broke up this weekend because he was being a dick and I said I was done and called my dad to come pick me up. We made up a few hours later though. This morning, I woke up and had a horrible gut feeling so I logged into his Facebook messenger (he gave me his information long ago) and I see he texted the girl I don't like. That doesn't bother me much, it's more of what they said to eachother. I talked to him about it to hear his side, he told me it was a joke. How is this a joke? It hurts. I just am so done with being hurt, but I can't bring myself to break up with him.. (blue is him, grey is her)