What a tough tough subject.
I think I learned the most from a close friend of mine over a drink when the subject got to her divorce. She hit me with the most sobering look and tone of voice and said something along the lines of, "if you knew what I know, you would find a way to make it work. Divorce solved none of the problems I was facing and just created a whole lot more of them"
Her saying this was probably more for her benefit as I was never even considering this for my marriage and she probably needed to relay the advise as a way to have some benefit from the hard lesson she learned.
You have to keep your eye on the prize which is personal happiness and roll with the punches as a good example for your kids. If it was not this challenge, another one would have been thrown at you. Showing a good example of making the best of it and persevering no matter what the challenge is will help you get through it, and be a strong role model for the kids as well.
What ifs are useful in analyzing the lessons learned but regret can be damaging. Whenever I am stuck and brooding, I usually find I am concentrating on the problem rather than the solution.
I hate to hear this but know that getting perspective of others does help.
The problem with my marriage was that only I was willing to work on it. He didn’t think he had any flaws, everything was my fault. I was originally planning on dealing with it until at least the children got older, but I decided I had enough. But in retrospect I solved nothing. I’m still dealing with him bc of my children. When you have children with someone they will be in your life always , especially until they become adults. And now I’m just suffering.