I'm relatively new on here but wanted to express my sorrow for what you've had to go through. That's a double betrayal which has to have a lot more sting in it - so my heart goes out to to you truly.
My only advice for "forgive and forget" is to get violently protective of both your presence and your sense of joy. Both your H and his OW have shown the world their hands. They've shown their lack of respect and integrity for themselves and for you. That's on them and maybe they will wake up one day and realize that they've erred in the worst possible way. You get to decide what you think about though.
Stay present. Whenever you think of her, think on how fulfilling your life is now. It's just like you said, she was the catalyst of your new life. As hard as it was, it happened and there are benefits. Focus on those benefits, list them out loud if you have to.
There's a great TED talk on alcoholism where the speaker talks about getting "the phone call" from her fiancé in which he leaves her forever, never to see her again. Her son made her list out the positives and negatives of what came of that action. Her negative side of the list had "I lost a guy". The positive side of the list was filled with stronger family bonds, more experiences and a whole host of things to be grateful for (I'm not doing this justice). I've made the same list and although some days it doesn't do much to alleviate the pain of loss, it's hard to argue with the tangibles of it.
My W left me and I got sober. Losing her is a hell of a pill to swallow, but every time I get upset with her for leaving, I think, "She didn't do this to you. She did this for herself and you turned it into the inspiration for the greatest change of your life. Focus on that instead of her."
Again, my heart goes out to you. I won't wish ill towards your H and the OW, but wish nothing but the best that life has to offer for you.
Hug back, PP
M 39 W 36 T5 M3 BD - 1/15 Separated - Same Day Served 9/15 D finalized 6/17
