If you’ve had honesty and remorse and it stopped 20 years ago, and for the past 20 years he has been dedicated and committed to you and your children, it is far more likely he was a man in a crisis who made shit choices rather than deal with his issues. There’s a world of difference between this and a serial cheat. His issues are important in understanding what led to his choices, but understanding is not excusing. He was seeking validation and approval by getting flattered by women instead of turning to you. His issues caused it, not you, and his issues may explain it, but they condone absolutely none of it. He still betrayed you and treated you appallingly. Infidelity is plain wrong, no matter what drove it in the first place, no matter what issues he had, he chose the wrong thing to do.
However, if you want to stay together and you want to get through this, then your chances sound good. He needs to understand why he did it without blaming you, in order to change his behaviour. It looks like he has.
If you believe what he has told you, and you believe there was and is nothing more to hear, it will not help you to pick it all apart and listen to randoms who don’t know him or you, trying to infer that there was more to it. You have asked the right questions and believe the answers you were given. They know less than you do. What is important is whether you believe that it was over years ago, he learned his lesson and it never happened again. The ‘he has shown you who he is’ comments apply to serial cheats. He hasn’t done it again, you believe, therefore his confession showed you who he was, not who he is now. No human being should be defined forever by their past actions, as long as they have stopped, learned from it and changed. What he showed you at the time was that in a crisis he made utterly shit, self-medicating choices rather than support you and your children. He has learned from that experience and is quite rightly ashamed of himself and wishes he’d never behaved in this way. What he is, is a normally decent man who was struggling and behaved in a selfish, cowardly and avoidant way, instead of dealing with it and talking to you. He regretted it totally and learned from it, changed his behaviour and you say he has been a decent, loving man ever since. That’s who you think he is, and that’s all that matters. However, this information has hit you like an oncoming bus. This is normal, you are playing catch up with it all, he has had years to process what he did and you haven’t. Look at the affair recovery websites and find the parts that resonate with you. What you are feeling is absolutely horrible but absolutely normal, no matter how long ago the thing that traumatised you happened. You are still processing it, things you pushed down at the time are coming back to haunt you. Read all you can about affair recovery and see a counsellor if your mental health starts to nosedive or you feel an objective ear would give you clarity. As you process it all, be prepared to find however that you can no longer stay with him, getting through infidelity and betrayal is hard and there is no shame in leaving or staying, what is important is that this is your life and your choices. Advice is just that, advice. It doesn’t mean it’s the right advice for you. Wishing you lots of luck, this stuff is so painful and hard to deal with.