is it stupid to be this sad over my parents divorcing?

My sister (17) and I (15) have seen my dad texting multiple people hearts and weird over the past 1 and a half years, we weren't 100% if if was him cheating or if he was texting his ex girlfriend (they had a kid together and their kid passed from cancer so they talk sometimes). Anyways, we brought it up to my mom multiple times and he fought with her everytime.

Well this morning, my mom figured out the password to my dads ipad and saw texts with a woman and she figured out he was cheating on her, not in person but online. I knew a divorce was coming at one point as they have been un happy for years but I was not ready for it. I feel so stupid as it is their relationship and not mine. But, I sobbed in my moms arms when I first figured out because I am so devastated. I still live with them as I am a teenager. I have struggled with mental health for years after some trauma and a couple months ago, he promised me he would try with their relationship if i tried to get better, but he said this all while he was actively cheating. I am absolutely disgusted with him, I will never look at him the same. I'm so upset and I feel like it's all my fault, my life will never be the same and I don't believe in love. He straight up admitted that he hasn't felt a sexual, emotional, or physical connection with my mom for two years and said he has been cheating the whole 2 years. I am in utter shock. Why do I feel like this if it isn't even my relationship? I can't believe this is actually my life.