The sentence that stands out. It would still be going on. So if you hadn't done the investigating and confronted him, he would be with her still. He lied about betraying your marriage vows. He has done it once, the next time he won't leave clues. You have to decide whether you can live like this, perhaps you can rather than split up. You might feel it's too late to go on your own, this marriage is better than the alternative. They are brutal questions to ask yourself, but you have to decide just what it is you want.
I feel for you in this situation, many years ago, I was, I finished it. Took a few years to recover, then I met the love of my life. I had age on my side, some of the people I met back then, in a similar position, didn't meet anyone. There's a lot to weigh up, but be honest with yourself.Good luck.