Four years of lies, one text message

AITA for going through my fiance phone and finding out her best man was actually her ex boyfriend who she never stopped sleeping with. I know going through phones is wrong. Just hear me out first. I am twenty nine years old and I was three months away from my wedding. My fiance Rachel is twenty eight and we had been together for four years. Everything seemed perfect. We had the venue booked. The dress was ready. My tuxedo was fitted. I thought I had found the person I was going to spend my whole life with. Rachel had a close friend named Derek who she always said was like a brother to her. Derek was actually going to be a groomsman in our wedding, which I thought was a little strange since he was her friend and not mine, but I went along with it because Rachel insisted he was family to her. They grew up together. They went to college together. She told me they dated briefly in high school but it meant nothing and they had been just friends ever since. I believed her completely. Three weeks ago I was charging my phone and grabbed hers by mistake. A message popped up on the screen from Derek. It said I cannot stop thinking about last night. I froze. I told myself it was probably nothing. Maybe they hung out as friends. But something in my gut told me to look deeper. I am not proud of what I did next. I opened her phone. Her passcode was our anniversary date so it took two seconds to get in. What I found destroyed me. Months of messages between Rachel and Derek. Not just flirty texts. Actual photos. Plans to meet up when I was at work. Messages saying things like I wish you were marrying me instead and this has to stop before the wedding but I do not know if I can quit you. My hands were shaking so hard I almost dropped the phone. I kept scrolling and found out this had been going on the entire four years we were together. Every single trip she took for work. Every weekend she said she spent with her sister. It was Derek. The groomsman. The man who was supposed to stand next to me at the altar and hand me the rings. I sat on our bathroom floor for two hours just staring at the wall. When Rachel got home I was still sitting there with her phone in my hand. She saw my face and knew instantly. She did not even try to lie at first. She just said Please let me explain. I asked her how long. She said it did not matter. I said it mattered to me. She finally admitted four years. The entire relationship. Every time she said she loved me she was also telling Derek the same thing. I asked her the question that still haunts me. Were you ever going to stop. She looked at me and said honestly I do not know. That answer broke something inside me that I do not think will ever fully heal. I told her the wedding was off. She begged me to reconsider, saying it was just physical and that she truly loved me more. I laughed in her face. How could I believe anything she said after four years of lies. Then came the twist I never saw coming. Derek showed up at our apartment that same night. He was not there to apologize to me. He was there to tell Rachel he loved her and wanted her to choose him. In front of me. In our living room. Apparently he had been planning to confess his feelings after the wedding, thinking she would eventually leave me for him anyway. I stood there watching two people who had been lying to me for four years fight over who got to keep lying to each other next. I walked out that night and stayed at my brother house. I canceled the venue. I canceled everything. My deposit was nonrefundable which cost me almost eight thousand dollars. Rachel has texted me every single day since asking for one more chance, saying Derek meant nothing and I am the one she truly wants. I have not responded to a single message. My mother keeps telling me I dodged a bullet and should be grateful I found out before the wedding instead of after. of me knows she is right. But another part of me keeps replaying four years of memories and wondering if any of it was real. Now I keep asking myself if I was wrong for going through her phone in the first place, even though it led me to the truth. Was I the asshole for invading her privacy, or did finding out the truth justify everything that happened afterward.