Losing yourself

After three years, my boyfriend and I broke up and now we're sleeping together...constantly. He got a new girlfriend and we're still doing it. And she's not even a cool girl. And knowing that she cheats on him annoys me because he's doing it to her. And I keep leaving but he always seems to find some way to come back and I take him back. I feel like he doesn't even care how that makes me feel... I mean, what am I to do now when I'm lost with him and lost without him?