I know a man who was hit across the back by a log at a remote logging work site. He was airlifted out, taken into surgery, and woke up with an incision running up his middle like a zipper. The doctor came in and told him that he was lucky that he had not lost his kidney. The man said, “Lucky H#$L! If I was lucky, I wouldn’t have been hit by a tree!” It took him months, but he eventually recovered completely and died in his own bed, an old man in his late '80s.
In that context, I will tell you this: You and your husband, in a sense, are lucky. He is not a serial cheater–I mean, he was faithful for over two decades and he has been faithful since his period of infidelities. He* is* capable of fidelity. He does have the character and the desire to go back to being a faithful husband. He made some very bad decisions at a very vulnerable time in his life, but through counselling he can learn how to avoid that in the future and through studiously avoiding the near occasions of this sin he can eventually deserve to regain your trust. You have not picked up any infections and there are no illegitimate children compounding the need for mercy and compassion. You can get through this and you can move past it. You can forgive him, you can work past the pain, and he can forgive himself, too.
I would suggest a book for you: “How to Forgive: A Step-By-Step Guide” by John Monbourquette, a priest and psychologist. I don’t mean you’re going to find every word of it something you’re going to follow to the letter, but it has some useful concepts in it. It might help your husband in coming to terms with what his dad has been doing to him, too. There are probably other fine books out there, but this is one I’m familiar with and have learned a lot from. It includes examples of how to deal (and not deal) with infidelity and with wounds coming from unreliable family members.
Don’t expect yourselves to get by this in a day. This is a process. It is going to take a while to wrap your mind around what happened, what your husband did, what has been done to him, and to sort out the complexity of it. Remember this: the main thing is to rebuild your marriage (which will be different than before, both for better and for worse) and to let go of resentment and unrealistic expectations in favor of forgiveness and a more serene acceptance of reality.
Honestly, you sound as if you are doing all the right things. It takes awhile, that’s all. If your house is flooded by some big storm, it takes awhile to clean up. There will be some mourning over the losses that can never be replaced. It takes awhile to decide how to prevent damage the next time a storm comes through. It is expensive, it is hard work, but what else are you going to do? You can’t live in the past. You can’t spend time obsessing about the future. God’s grace is all in this moment and in what you have to do right now. With the help you are getting, you’ll be able to return to living there. You have an excellent foundation, even with the earthquake you’ve been through. Don’t be afraid to hang in there. Maybe you won’t succeed, there are no guarantees, but I would bet that you will get through this. Your path will be hard, but your prognosis is actually excellent. You have all the marks of a couple who will survive this, so don’t be afraid to take comfort in that thought.