Thanks for the clarification OP.
People often blame themselves for trusting, but the alternative option of snooping, hyper-vigilance and jealousy is ghastly. However, once your trust has been exploited, I always suggest that for a time, people give themselves permission to be on their guard and to reframe their mistrust as the most sane response in the circumstances.
If a partner deserves to be trusted again and has done everything to take responsibility and atone for the hurt caused, the mistrust goes and faith is restored.
In your case, it sounds as though your H has been passing the responsibility for his infidelity over to you entirely and has made this all about whether you make him happy or not.
One of the best gifts you can give yourself and your DD is to accept that there was nothing you or your relationship could have done, to prevent your H's behaviour. This is his problem, not yours. He will be unfaithful again, because he believes the myth that infidelity is entirely acceptable if a person is "unhappy". No doubt the OW in your case also believes this myth and deludes herself that if she is all things to this man, he will never do the same to her. She'll learn. He is already treating her with disrespect and cowardice, not loyalty.
Refuse to engage with her at all. Change your number or block her. With your H, if you find yourself having a conversation with him about his behaviour, tell him most firmly that this is his problem, not yours. That you have realised that this is not about you, or your marriage at all. That you know that there are decent men who don't have his particular problems and that you're very much looking forward to forming a new relationship at some stage.
Coming back to the start, what you have learned from this experience is that if someone hurts you and then does it again, there should be no second chances. That he also did this with yet another OW demonstrates this point most vividly.
One day you will look back on this past Christmas and celebrate the knowledge you gained. Yes, you will still kick yourself at times that you let him back in, but praise and celebrate your instincts that you realised very quickly that he wasn't up to the job of being in a committed relationship and you took action swiftly. That means that you are going to be okay. 