If you're actually asking how I would respond in this situation, its hard to say without more details. You said within 3 years of the relationship starting, and that it is a long term relationship, but you don't indicate how long. 5 years vs 20 years makes a huge difference in this scenario.
My thoughts on being cheated on are that it probably is at least partially my fault, I wasn't providing something that my partner needed in order to be satisfied with some aspect of our relationship, how much fault is on me could vary widely. But I lose respect very quickly for someone who isn't satisfied with what I bring into a relationship and cheats without having expressed to me what I am doing or not doing that is leaving them dissatisfied. In my mind, if you are in a relationship that is not explicitly open, then there is no excuse for cheating when you haven't give your partner and opportunity to change or an opportunity to end the relationship without getting hurt. I respect someone a lot more if they tell me I'm not making them happy or they need something else and end the relationship or give me the option to end the relationship, then someone who stays in our relationship for whatever benefits they get while getting their needs satisfied elsewhere and behind my back.
The length of the relationship and how much time has passed can change things significantly, and I've never been in a relationship long enough to get over something like that. But I don't know if I would be able to continue the relationship unless I could fully forgive the person completely, and I don't think I could begin to forgive them if I hadn't confronted them and discussed it completely. And, of course, there is a big risk of getting hurt more and hurting the relationship more at that point, but if a relationship can't survive that then I'm not certain that its' worth saving.