I feel like I'm talking a lot about my own experience here, but hope at least some of it is helpful or comforting to anyone else going through this.
I think some people are just more selfish. And some fear getting older. Definitely the case for ex-DH, who was approaching 40 the first affair and coming up to retirement from the job that gave him status the second time.
It's shocking and frightening to witness someone you love and trust turn against you so completely and suddenly. The second time he left I asked him if he realised this might mean he lost his children and did he think it was worth that, and he said yes, he wanted it whatever the outcome. I'm not sure that many women would be able to respond that way.
I was mid-50s when I remarried. (I was very young the first time, so maybe we did well to get to 30+ years together.) Of course, we didn't have to marry second time around, but it was something we wanted to do, and my DH in particular felt it was a good thing to do to show my children he was committed to me. Financially it was a terrible choice as it ended the pension arrangement I had (too complicated to go into here), but it was heaven to be free of the last bit of control that ex-DH had over me and messed around with every month, causing endless upset. (A bit of advice: do anything you can to maintain/acquire an independent pension source. Some schemes make it difficult for you to do this.)
I don't think anyone should feel ashamed to admit that they wanted to take an errant partner back. I suffered both times he left, and felt my life was over and the past was tainted. I felt unworthy and dirty. At the start I mourned the wonderful husband I'd lost. I felt it would have been easier if he'd died, as he wouldn't have chosen to leave me. They were dark days. It took time to move on from that. I read somewhere it takes a year to recover for every 7 you were together, and that seems about right to me. Eventually I got angry but most of that wasn't about his leaving or having someone else, it was about the way he treated me afterwards, and the control he wanted over how things played out. (Interesting how they don't want you, but they still want to control you.) I realised later a lot of it was caused by him proving himself to the OW. After all, if he was so untrustworthy he'd have an affair, he needed to work doubly hard to show his commitment to her, even if that meant being extra nasty to me. (I do wonder how people who've got together that way ever really trust one another, having both proved how untrustworthy they are?)
These days I don't feel hate or bitterness towards him. I don't like him either. I just feel indifferent. He's of no consequence in my life. I never thought I'd be able to say that. I can look back at the past and it doesn't seem ruined any more. I can see the good times we had. The relationship I have with my (now adult) children is fabulous. And the bond they have with my new DH is more than I could ever have hoped for, testament to what great people they are, and that they've been prepared to open themselves to it. So please, don't ever think your life is only worthwhile or complete with that one person who's disappeared into the sunset. There is a good life to be had beyond that. Be prepared to embrace it, literally.