Can I add another comment please?
I am nervous posting this as I don't want to seem like I am judging people when I am not. This is not to disrespect or offend anyone who choses separation or divorce, it's just an observation I have made about people in my life, friends and acquaintances.
Please please don't think I am judging as every situation is different. For many people it IS the right decision.
I believe a lot of people rush in to it and later regret it. Mostly when they have been together a very long time. I know many people who divorce, meet someone else, separate, meet someone else, separate, maybe remarry again, then divorce. They rarely seem to settle into happy relationships again. Although it does happen of course.
I have spoken to many divorced/separated people who deeply regret their decision. My parents for a start. My Dad died lonely after years of hoping my mum would go back to him whereas my mum has bounced from man to man, even returning to my dad twice in the early years. She eventually remarried a man she is not in love with but who looks after her and is reliable. Not a reason I would marry but each to their own. She has told me since dad's death that her biggest regret was not going back to him and staying.
On the other hand I know a few people who separated shortly after marriage, they both realised they were with the wrong person and they later settled happily into new relationships/marriages and have now been together for years. That seems to be more of a case that they weren't with the right person, rather than being with the right person but choosing to leave because of the tough times, if that makes sense.
My point is, that if you have been together a long time and were happy before things got rough, then I believe it is worth fighting tooth and nail to try and stay together. As long as the A is over of course and both parties are fully committed to R.
Of course if you don't feel you are with the right person it would be the sensible option to leave.
My thinking is that I could never love anyone as much as I love H, we have been together through thick and thin for many years. we are soul mates. I will not give up on that unless I have to because I truly believe I would regret it.
If he did it again though, well, that would be the time to call it a day, without a moments hesitation. Again, that's just my case though. I know everyone is different.
I hope this makes sense and doesn't come across as a load of twaddle lol.
ETA - pretty much what MandMS said, with my own thoughts thrown in.