Cheating dad, need advice

This is my first time here and honestly, I don't know how to start this. I have a loving family, my dad has a high-paying job and my mom loves me along with my brother unconditionally. It was all going fine until last Saturday, the 7th of October. I remember doing my homework when I heard my parents arguing, it wasn't new, they argue all the time but I decided to eavesdrop on their conversation for no reason other than to know what the conflict was about. My dad has been cheating on my mom with an employee of his, a woman whom I remember was just in her mid-20s. I can recall myself being relatively calm that day, of course, I was mad at him, but I didn't say a word to defend my mom. I know I'm in the wrong and that as the oldest daughter, I should be the one handling the problem. I know I should have spoken up but I stayed muted through the whole ordeal. I just didn't want to ruin what was left of our family and I desperately wanted to believe that if I let it pass, things would just return to normal and we would be happy again as if it never happened. I hate it when my mom talks with me about how my dad is completely infatuated with that homewrecker and if I were to play pretend and act like nothing was wrong, he would replace her with that woman. It was my greatest fear, to have my family fall apart. I told her I was sick of the pressure being pushed onto me, I told her school was tiring enough and the family matter was too much for me to handle. She didn't know I cry every night before bed, she didn't know it was hard for me, too. I'm going to confront my dad this Saturday(14th) about the cheating issue, I promised my mom that. I need help, I don't know what to say, everything has been out of control lately and I'm stuck. I know I'm a terrible daughter for upsetting my mom and I'm making up for it, I'm sorry, mom.