Oh @Bedraggledmumoftwo what a shock for you. You sound like you have been so supportive and trusting and have contributed such a lot to the partnership. I will tell you my situation as I suppose it may be helpful to know? Sorry if it's long!
I don't suppose I will ever truly know when the affair started in emotional terms and I don't know whether it was the chicken or the egg. I was pregnant with DS2 and my ds1 stopped sleeping well and we started arguing a lot about his sleep. Then DP did some major DIY around the house that took up all of his spare time for about a month, once that was finished he wanted to spend his spare time pursuing his lifelong hobby and wanted to get out of the house because we were arguing so much about everything at this point. I became very anxious and he got frustrated with me but was not helping me out round the house or being emotionally supportive at all. We had drifted apart and were not a team. I do acknowledge this.
Little did I know that someone had started at his work who liked him and made it known to everyone. Apparently he at first found it quite awkward, then they became friends, then flirted and then kissed just before Christmas, he liked the attention. (This is difficult for me because I was very low at this point and felt so alone.) he had by this point become an absolutely nasty, critical and controlling wanker to me, which he says he thinks was out of guilt and stress of keeping the secret- poor, stressed, lying DP! He told me he was working late on a few occasions over a few months when really he was going to hers after work and I just believed him. I started counselling for my anxiety and depression in January and started to feel better, I started to really try with our relationship but he was very resistant to all my suggestions. In Feb (when the physical affair had been going on for a couple of months) I saw an advert for his dream job and told him about it. He started the application process which meant he was out even more (legitimately) and had a lot of work to do, I diligently supported him and took on even more of the home life work load (post c-section). He secured the dream job, was waiting for references from his boss and BAM! His boss said someone had noticed something and they needed to investigate it at work before doing his reference. So my partner came clean because he knew there was a likelihood I would guess with all the stuff at work. He resigned the next day and has been at home since. The investigation amounted to nothing so he is starting his new job soon.
He said that he told himself that it wouldn't harm anyone because I wouldn't ever find out and they had stopped the affair a couple of months ago because he had been feeling so guilty. He thought that the new job was his opportunity to leave the affair behind and the start of a new chapter for us. When I found out I said to him that his behaviour had been so bad that I had been contemplating ending our relationship, especially as all my efforts to repair it were rejected, I showed him tons of descriptions of his manipulative, mean behaviour and he was horrified. He said he could see retrospectively he was doing that but couldnMt believe how bad it was. He knows that the affair was actually harming me all along through his behaviour.
Since I found out I have actually had a massive sense of relief that I wasn't going mad and that he really was that horrible. And also a relief because it is literally like he has snapped back to being the person I knew before, except that before I would never have thought he would do what he has done.