I think if you all work really hard at it, you might get over it, but he needs to realise that it's not going to be easy for you to trust him again after he betrayed your trust for so long. He needs to realise how disrespectful of you he was and take responsibility for it rather than play the victim ("it was so hard for me!").
I think he should be more afraid of rushing and ruining everything than of this other woman moving on. Not to mention, maybe that woman is fine with being a mistress but wouldn't be fine with being in a poly relationship. Does he know where she stands? Does she know that you know?