Hi OP. I found out just over a year ago that my DP of 17 years was having an affair that had gone on for 18 months.
Our situation is probably completely different to yours. I withdrew from the relationship prior to the affair starting, didn't want intimacy/friendship etc and was used to him being bothered about it, asking me why, trying to talk. This is where I take responsibility (partially) for the state of the relationship.
I first started noticing something had shifted when he appeared less bothered. That gradually turned into resentment, shouting, blaming me continually for everything, staying out for the night because 'his friend needed him', to me holding the door open for him and asking him to stay away half the week as I couldn't put up with his moods. Eventually, I had an anonymous phone call from a man telling me what had been going on.
DP walked in about 3 hours later, completely oblivious. I confronted him, he denied it for hours.Then it came out. I shouted, screamed and kicked him out. Changed the locks, kids were in bits (they luckily had stayed in their nan's the night before so didn't witness any of this). He never said he wanted to go, never said he found her more anything than me. Just kept saying sorry, he was lonely, needed someone, nothing serious.
I spoke to her - she saw things differently. Said they were getting married, pretty much. He has always maintained that it wasn't 'real'.
I found out all sorts of things that I never thought he was capable of.
But after living apart for a month (he didn't move in with her although I know he stayed with her for one night, a day after I threw him out), and lots of talking we got back together.
It's been hard at times - the pain is incomparable to anything I've ever known. But we're over a year on and things are great. We didn't have counselling, we've had a few dark days where the pressure was through the roof (I sometimes asked for answers that he cant give) but the love we have for each other is real and was worth saving to both of us. His apologies, actions and remorse have helped.
At this point, I don't wake up or go to sleep thinking about it. He does everything he can to keep me feeling loved and re-assured.
The low point after he came back was the OW acting like a crazy. I contacted her boss. She tried to take me to court. He arranged to meet her behind my back TWICE to appeal to her better nature and ask her not to. He told me afterwards but I wasn't happy to say the least. And she still tried to take me to court - went on for 6 months before she stopped trying (she didn't have a leg to stand on but her solicitor was clearly making a fortune out of her - I didn't even instruct one, it was that stupid).
Sorry for the hour-long confessional - other peoples experiences help/helped me through a very difficult time. If you're both committed, it can work. And (unlike me) try and keep resentment towards OW out of your head - it only really hurts you, doesn't solve anything