Yes, he's cheating with her. And it's not about loss of love for you - it's about loss of respect.
One of the problems is that expectations of him are extremely low, so there must be an element of you not respecting yourself adequately. Problem is, if we don't show respect for ourselves with appropriate boundaries and expectations of those around us, how can they respect us in return?
Time to start showing yourself some respect. Figure out what is appropriate in your life and don't accept any less. If that means kicking him to the curb, then so be it. But he will never respect you while you don't respect yourself.
Edited to add: his behaviour is not your fault. He sounds like a Class A abusive dickhead. But the behaviour continues because you continue to support him no matter how horrible he is to you.
Last edited by basilandthyme; 02-06-15 at 03:38 PM.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.