dump him, then forgive him. Probably easier said than done but forgiving someone is the best way to move on but certainly don't forget. Forgive don't forget.

9 years? Tell him to go f*ck himself. I would also punch him.

The truth of the matter is that forgiveness is about YOU! Weather or not you forgive him or not has nothing to do with taking him back. As long as you don't forgive him you allow those feelings to rule you. Forgiveness is about taking the power back. Once you forgive you release all those negative feelings and are able to start moving past this instead of having to feel that pain and anger over and over again. Why be held hostage emotionally? As to taking him back or not, that's another choice you'll have to make. If he is getting help and you actually do see change, you could give it a try. If you just can't get past what he did, move on. You have to do what's best for you.

Truth is, you can't forget it. You can choose to move past this, you can choose to let him work towards earning your forgiveness, but its not an easy rode. You have 20 years together, and, aside from the infidelity, you don't mention any other problems, not that I'm saying cheating on you for 9 years is no small thing. Its pretty huge.

Your best bet, if you can't figure out if you want to give him a chance or not on your own, is to go see a counselor or therapist yourself and work with them. Thats actually a really good idea either way. Still you need to decide ultimately for yourself if you want to give your marriage a second chance or not. His putting himself in thereapy is a good sign.

I absolutely would not forgive him. How can you trust him again? In my opinion, there's no going back after you cheat, and wow.. for 9 years?! Are you kidding me? Don't even consider taking him back. You can find someone better, especially if you're so forgiving that you would consider staying with him.
Good luck!
-M.E.