There are two reasons for divorce I have seen, one people marry too quickly and are simply incompatible (a lot of divorces during lockdown happened because people were forced together and they cannot stand each others company for long). Other is thinking that once the ring is on her that the game is won no more effort.
It takes two to tango. You can do everything right and your partner may still cheat on you, manipulate the children to dislike you, etc. It is unfair to say "you married too quickly" (apart from shotgun marriage, at some point you must make a leap of faith after all) or your second point that this person must not have tried hard enough. There are many reasons a marriage can fail, an many of them are out of one's control.
Absolutely, some people will roll triple 1 and land their feelings on absolute garbage of a human and will get hurt.
I think the point I was trying to make was that one of components of long term marriage is that you need to keep it up (both sides need to keep it up).
My closest friend I've known for 10 years and we know each others intimately, it's a deep connection that can only be built with time and effort. It's without comparison to other friends that I've only known for a few years. So you spend 30 years building something so strong with a single person, there's never going to be someone else like this, you can't spend another 30 years building something with someone else, that's it. I don't know how you can deal with losing something like this.