I've actually named changed for this LONG post. Been married 30 years now. Both in our mid 50s.
About 20 years ago and after 10 years of marriage and 13 years together and with 2 primary age children, my husband (who had mood swings and was arsey with me for no reason some days anyway,) had an affair. He suddenly started to go very distant on me, didn't want sex, and seemed to be constantly messaging, emailing back and forth, and talking on the phone two or three times a week to a woman at work. I'll call her Caz.
She had a partner she'd been with for 10 years (no kids) and she was really unhappy with him, and my husband was being the 'knight in shining armour,' and 'a shoulder to cry on,' and was spending lunchtimes with her 'consoling her' and even though he finished work at 4.30pm (and so did she) he would often not come home til 6.30pm. And his phone was always switched off. I kept saying 'I'm really fed up with the amount contact you're having with her.' He said 'she needs me as I am her friend. I'm supporting her...' 
Long story short. About three or four months after I first became disturbed about the amount of contact (and about six months after he first started 'consoling' her,) a message popped up on our joint computer from her on his email. He had gone to the loo and left his emails open. It went BONG, and yes I did look. It was from 'Caz' and she said 'thank you so much for last night, and being with me, it meant so much to me. You mean the world to me, and I'm going to go to sleep dreaming of you. Nobody has ever made love to me like you did. I think the absolute world of you, darling.' I felt like my whole world had crashed and burnt, and went completely cold, and felt like I was going to be sick.
He came downstairs, and I went absolutely batshit. I confronted him straight away and I said 'you have fucked her, you fucking bastard pig!' And he hung his head down, and said 'yes, we have had sex three times. And yes to last night. I said I was working but I was with her in her car.' (That's where they fucked-classy.)
And I said, 'do you love her?' He said. 'I don't love her, but she needed me.' It's not going to happen again. And I won't be seeing her again. I regret it and will end it.' I said 'of course you will see her again, you bloody work with her.' I said. 'You or her are going to have to get another job.' And he said 'I'm not getting another job and I doubt she will leave either.'
So, I said 'well then. You're going to be looking divorce papers in the face in the next few weeks, because there's no way we can go on after this, and with you working with HER still. I don't believe you will end it. You are a liar and a cheat.' He just sighed...... Also. I knew her phone number because it was on the itemised bill, so I said I'm going to ring her partner and tell him that you've been fucking HIS partner.' He actually blocked my way and STOPPED ME using the phone to stop me telling her partner..., he said. 'I'm not having her hurt, and will not let you harm her in any way.'
So it seemed like he was really concerned about her being hurt, and that absolutely fucking devastated me. She (and my husband) had dropped a bomb on MY life, but I was not allowed to do the same to her?! 
Anyway, when he realised I was going to definitely divorce him, and was about to start the process, he was suddenly very contrite and very sorry and absolutely devastated... He went to the doctor and got four weeks off on the sick with stress, because he said he can't face her and he doesn't want to see her again and he will look for another job. He heard after three weeks off that she'd actually left. She'd been there for about a year. She's only 31, so it wasn't too hard for her to get another job.
He wanted to start again, but at that point I was so angry that he'd been with another woman. Being emotionally connected with her, knowing I was suspicious and upset, and then having sex with her, and then practically knocking me over to stop me from telling her partner. So I was done. I said 'I don't think I WANT to try and make it work.'
At this point (a few weeks after,) I really hated him, and I actually went online and went on to contact sites to find different men to hook up with. I thought 'you're not the only one who can fuck other people whilst married...' I didn't want to try and make it work with him at that point. I met about six or seven different men over about three months and has sex with three of them. One of them, about five or six times. He knew it was going on and he was devastated I was doing this to him. I thought 'it was alright for you to do it to me though.'
Anyway, long story short. I stopped after a few months and he seemed so devastated our marriage is apparently over, and he really wants to make it work. So we gave it another go. About seven or eight months down the line, we renewed our marriage vows.
This was about 19 years ago. And he was a completely different man after that. He treated me so much better than he did for the first ten years of our marriage, when he was quite cold and distant quite often... And if I said the 'wrong thing' he would go silent on me for a day or have a go at me. AND he kept all his extra money/overtime money to himself. He earned more than me because I was part time because of the children, and I often had nothing at the end of the month after paying 'my share' towards the bills, whilst he had 100s left. Since all this happened, (and we renewed our vows,) the last 19 years have been happiest marriage I could possibly wish for.
It was devastating at the time though, and I have never felt pain like it. He felt the same pain and anguish when I was fucking other men. Not ideal, but we worked it out in the end. I believe most people have affairs, but yeah, they are very damaging, and I don't know why they do it. My husband never said why, but mine was revenge. I know not healthy or ideal, but it's done now, and we are really happy. Yes, we really are...... 