(((hugs))) Hell, my FWH and I are R and are working hard to grow closer every day, and I still lose it one way or another when I see couples like you've described. Infidelity kicks us where we're weakest, and continues to give us a kick randomly for good measure for a hell of a long time. Being uncertain is OK. Feel what you need to feel.
He is in therapy addressing all his issues. But unfortunately I can't see if there is progress and I'm not sure how he can prove it to me. He is living back in his home state with the in-laws.
If you really want to know how he's doing in therapy, ask him to sign a release with his therapist for full disclosure to you. Then setup a time and either go back there for a visit and an appointment alone with his therapist, or stay where you are and do a phone consultation with the therapist. When my FWH was exposed in a huge lie to me, I told him that the next time he went to his IC, I was going as well and his IC was going to answer all of my questions or we were through. And believe me, I grilled the IC. If this is what you need to be certain in your mind of what you need and want, then do it. Certainly a negative reaction from your WH will speak volumes to you.
Look. You're going to second guess yourself a lot. This is a big decision so that's normal. But you have to look at the likely bottom line. Is it likely that your WH can make the changes that you need to be a worthy partner to you? Is it likely that IF those changes are made, that you can live a satisfying and enriching life with him? Do you want to spend more years living with someone that, given his track record, could break your heart into pieces again? In two years, can you see a future where you are healthy and whole, possibly with someone who loves you and acknowledges how lucky he is to have such an amazing woman in his life? Just like that elderly couple.
Sometimes you just have to acknowledge that your decision is for the best, and that it's going to hurt, but that a bit of hurt right now cleaning out the abscess is better than letting the festering wound grow bigger and bigger and bigger.