I think I have to agree with Scotty. He either fell for that girl, or else he's just always walking out with her to make you think that he HAS moved on. But whatever, his reasoning is certainly questionable to say the least! He should have manned up and told you the truth! Leaving it like this stinks. It makes him look like an idiot -- which he obviously IS.
I hate to say you're "better off" because I don't know that. I'm sorry you're hurting. These things are never pleasant, but the old cliche' that time heals is true. Time DOES heal the wounds, but the waiting is a killer.

All I can tell you is to keep busy, go out with friends, and perhaps read or get involved in a hobby, etc.
God bless and you'll be in my prayers. Take care. Hugs, Lee