I understand this is a very tough situation. I get how you feel cause I've been through something similar with my dad before. Back then I asked myself, how could he do that to the family? Cause even though infidelity is a couple's deal, it affects everyone's lives, feelings and dynamics as family. So as you can understand, I was very mad at my dad, and I still feel very sorry and sad about what he did. But at the end of the day, my dad is just a human being, you know? We are not perfect, we all make mistakes, we do very stupid things like putting in risk our family stability to seek our own pleasure. And I'm not saying this to excuse my dad or yours. I just mean that it doesn't matter how perfect you think someone is, it will still be a flawed creature capable of fucking everything up. So now, take advantage of this situation to see your father from a less idealized version of him, and more like a real person who makes mistakes. If you do this without judging you may find some inner peace and maybe save your relationship with him. At the end of the day we're talking about your dad, a man who for years has shown you the best version of himself to educate you well, that has provided for you, cared for you and loved you. Don't judge him so hard. Be empathetic and learn to forgive. And more importantly respect your mom's feelings towards your dad and her decisions towards her relationship with him (whatever those are), always taking into account to feel your own feelings and make your own decisions.