Most of the advice here has been good, apart from one person a few dicks who have an overly simplistic view of the world and think they have some moral authority
You don't know the details of your parents' relationship. You don't know how they behaved when you weren't there
Life is never black-and-white. This isn't condoning cheating, it's an acknowledgement that it's very simple to say what's right and wrong when you are on the outside
What's shit is that you are being put in the middle
The problem is between your mother and father and neither of them should expect you to take a side, but once again, life is messy and hurt people do things in their anger and frustration that aren't rational or reasonable
Time definitely helps, and things can take years to settle, if they ever do
It's unfair for you to be used as a pawn in their game. People who make demands and try to squeeze you with a "you're with me, or you're against me" aren't looking out for you, they're looking out for themselves
No child should be weaponised.
If you can handle the conflict, or potentially messy details, you can ask her why she expects you to abandon your father. She might come to realise what an unfair position she's putting you in
Either way, good luck, and remember that it's easy to judge, but there's a lot of truth in the old saying about walking a mile in someone else's shoes
There was so much fucking awful content there for so long
I laugh when people say that it's "leftist", because it sure as fuck wasn't, and isn't now either