OK, so here's a suggestion, which you can see whether it works for you or not...
At any particular point in life you make the best decision you can, given the information that you have. The result may or may not turn out to be a desirable one.
The goal of "learning from your mistakes" is not to beat yourself up with "if I'd only known.." or "Why didn't I..." type statements. The goal is to get more information based on your experience, so that when you put yourself in a similar situation in future you have better information upon which to base future decisions.
You made a good list in your original post about the various things you did. Go through them and label each "I did this right" or "I probably shouldn't have done that, knowing what I know now". Try to come up with what you think you would do differently that may cause a different, more desirable outcome.
Do that analysis and take this all on-board and then try again, and don't obsess over other things. You both went into this as adults, knowing that there was a chance it wouldn't work out. If HE can't move on, then that's respectfully not your problem, but his.
"Listen, or your tongue will make you deaf." - Native American Proverb