Why is it so hard to leave?

Every time my husband & I fight he messages other girls. Nothing crazy just making conversation, but when I bring it up it gets turned around on me & im apologizing & begging for forgiveness. He goes off on me, calls me crazy, says fuck you, that he’s taking our son. But I apologize...I’m just so mentally tired & exhausted from it, I live in constant fear that he’s cheating on me. I’m just tired..,