Why can't I let him go ?

My ex & I been knowing each other since 2013, He left me heart broken in 2014. Met another female, and now she's pregnant. After everything I still love this man and I don't know why when be out me through so much hurt & pain. I even tried to over look him having a child with someone else & working it out but it just went down hill. And now where back communicating. I honesty just wanna move on but I'm so in love with him. I know I deserve better but I'm just used to this man.