I think the sad reality is it never goes away. It will always be there in the background. The question is can you live with that.
It helps if your DH is genuinely sorry and doing everything he can to make you feel safe and comfortable. Is he open about everything without you having to ask? Does he comfort you when you feel down, stand by you, cancel any work travel or nights out if you are uncertain about them? Is he in short falling over himself to keep you?
If he is then that is a good basis, and perhaps enough to help you move on. If he's not then my advice would be to tell him to move out. If there are any doubts as to his sincerity it can't work in the long run, because living with constant doubt is no life.
The OW means nothing to you. Write her off for what she is. Someone who lacks morals, decency and any form of empathy. I'm not so sure why she feels she has anything to gloat about. A sordid affair, that ended. Not something I'd brag about myself.
I feel for you, I know what this awful uncertainty feels like. I've been going through it myself recently. I don't know why but last night I had a light bulb moment and just let it go. I have a chance at a happy future with a man who has shown that despite past mistakes he is truly sorry, and would do anything to keep me. And I still love him. So, I'll spin the roulette board of life and take my chance.
If you love your husband, he loves you, and he is doing all the right things then ask yourself if you are willing to take that gamble too.