As a person who also witnessed their own Mom's affairs as early as Kindergarten....

Get help now with a therapist. Make your parents pay. Your mom's choices are NO reflection on you. It is not even about your Dad. It is about her.

I would write a letter or two to your mom and destroy it. Find a therapist and handle it. Then invite your mom to an appointment and confront her with a professional. I say this because my own mom denied it when I confronted and gaslighted me. It made me question myself and my own truths.

I'd make your mom and dad pay for weekly therapy honestly.

Their marriage isn't your business and most importantly NOT your burden. One day you'll understand. But please realize this has nothing to do with you and your mom's decisions to seek affairs is her own choices.

I'm sorry! Wishing you the best of luck.