Found out 4 years ago that my mom(56) was cheating on my dad(57). Just told her that I know and she is ✨gaslighting✨ me(23F)

Some background: I live at home with my parents. My parents are super conservative and recently we had a talk about how I shouldn't stay at my boyfriends after spending dinner at my house. My mom said it "gives her the creeps" which is short for, she thinks I go over to his to have sex. NGL we don't abstain from sex but 9 times out of 10 I go back with him because I love him so much and want to cuddle all night. And my parents didn't care at all when my twin brother lived with his girlfriend at her families over quarantine.

So, along with this talk we had an argument. I have been resentful of my mom for many reasons, and she has noticed and asked me to talk about it with her. I mentioned how she doesn't respect mental health, abused us as kids, was super controlling, and was mean and emotionally absent throughout childhood. We got into a heated argument about whether or not spanking(with wooden object really hard) is ok, and I got so frustrated I went to my room. I texted her that I know she cheated on my dad and that this is the biggest reason I resent her. She responds that she has never done that and she told my dad that I accused her and has no idea where I'm getting it from. She comes into my room and starts gaslighting the hell out of me. I told her I saw proof, I saw her texts with the other man saying "love you" "I need another overnight with you" she even talked about our family to him). She's saying I'm making it up and keeps insisting she has no idea what I'm talking about. I could not get her to accept that she did it.

This was last night. I don't know what to do now that my dad knows of the accusation. I only know that she talked to him about it, I didn't talk with him last night. Im assuming she told him that I'm crazy and just trying to cause issues. My dad is very sweet (besides spanking us as kids), he has been more emotionally present than her. I feel so bad for him and am not sure wether I now have to go against my mom to prove her wrong or to let it go for the sake of keeping the peace. Im beyond upset with my mom that she is out here sleeping around, and has the nerve to tell me to have "good christian values."

For the last 4 years, I've had no idea how to go about this. Nobody knew accept for me. I figured that going straight to my mom about it was the best thing to do, but now it seems like its going to be an ordeal with the entire family. (I am one of 5, and one of my brothers is super loyal to my mom)

What advice would you give me for how to go about this without making the situation worse?