My DH had an affair (a long one, 18 months) whilst very depressed having lost both parents and his job, dependency on strong OTC painkillers and seemingly on self destruct.
He was unrecognisable in every way for a couple of years - eventually he came and went as he pleased, full of bile and anger. I didn't know there was an OW but when I found out I felt relieved - obviously incredibly hurt and angry too but so much made sense when it came out.
I know my DH very well after 21 years together and those 2 years were like living with a stranger, as if someone had cloned my DH and one day I woke up next to him and that was my lot.
Very odd, strange, disorientating and I would not ever like to go through that again. The DCs were (and always will be) my priority.
When it came out he was immediately a mess - just broke down completely and couldn't cope. I kicked him out and changed the locks. He went to a friends. After a few weeks I agreed to help him get off the prescription drugs, spoke to his GP. Around 8 weeks later he moved back home and we worked through it.
The OW in this case, although long term, was on-off over the period. She was married, then going through separation, then met someone else - it was all very Kyle-esque behaviour by both of them, but that's not the point here so I won't go into detail.
We're over it but this was more about what was happening in his head, he really was on self destruct for lots of reasons. I chose to accept that under normal circumstances and without those conditions, he wouldn't have done what he did to our family. If I did not believe that, I wouldn't be with him now.
(Before anyone tells me I'm making excuses, as happens on so many other reconciliation threads, please remember that I knew him for 17 years prior and am in a good position to make my own character assessment on my husbands behaviour - even if you think you know best. We're almost 4 years on now and very happy.)
I hope this is helpful, OP.