I have been the 'ex wife' in this scenario. My H left to put himself first, gave me all the classic midlife crisis bullshit about needing to be alone for a bit, didn't know what he wanted long term, just wasn't in love anymore etc. Note, I have a full time, high earning professional job, as well as doing everything for kids etc for most of their childhood. But my job is home based, his isn't, so it made sense for most of it to land on me. And that was ok for me. I knew that this was the reality of the parenting years and I was looking forward to us getting back to us again. I thought we had a broadly solid relationship that had ups and downs like most.
He very quickly got involved with someone else, yes the good old cliché, over a decade younger. Bought an entire wardrobe of new clothes, changed his tastes and interests. Did all the things that you can do when you aren't looking after your parenting responsibilities etc.
I had a lot of legal advice from some amazing strong female lawyers who all rolled their eyes and asked if I would have him back if he asked. I said yes, so we agreed that if he wanted a divorce, he could push it forward. I don't need him financially, I am completely self sufficient. Had a similar conversation and response from an HRT specialist, the number of men she had seen do this.
About a year after he left, he started looking at me differently whenever he came over to see the kids, who wanted nothing to do with him. He seemed confused by me. A few weeks after that, he ended his other relationship and asked if we could start again. It took another few months before he moved home. I won't say it is perfect yet, we are still in recovery phase. But we are in so much better a place already than we probably had been for years. I am not going to say it has been easy, but I think of his time out of the house as having had a sabbatical, gave us both a chance to reset.
Long winded way of saying, don't be so sure that he won't go back home. If he really wanted out of his marriage, he would have started proceedings. A lot of men really don't like the reality of what they have to give up financially and decide that maybe the 'ex' wasn't so bad after all.
I know this is not a popular approach on MN, but I just got on with living my life (which has meant I now have lots of new friends, hobbies and interests), left him to his. And let time do it's thing to his fling.