They Planned Her Breakup Together

AITA for confronting my best friend after discovering she was secretly dating my boyfriend behind my back I thought I knew everything about my best friend Lily. We had been inseparable for over five years. We shared our secrets. We trusted each other completely. Or so I thought. It all started when I noticed my boyfriend Mark acting strangely around Lily. He would avoid my gaze. He seemed nervous whenever she was near. I brushed it off at first, thinking I was overthinking. But then I found out Lily had been secretly texting Mark for months. It was a casual message I found on Mark's phone. Just a casual hello. But the timing was suspicious. I decided to dig deeper. I installed a messaging app on Mark's phone when he was not looking. What I saw made my blood run cold. Lily and Mark had been exchanging long, intimate messages every week. They planned secret meetups. They called each other pet names. I could not believe my eyes. My best friend and my boyfriend were secretly dating behind my back. I felt betrayed, humiliated, and furious. I could not believe Lily, my closest friend, was capable of this. I waited for Mark to come home that night. I confronted him first. I showed him the messages I had uncovered. He froze. His face turned pale. He tried to deny it at first. But I had proof. I told him I wanted the truth. He broke down and admitted everything. He said they had been feeling lonely and misunderstood. That they had fallen into a trap of false comfort. Then I turned to Lily. I called her immediately. She answered with a nervous voice. I told her I knew everything. That she had been lying to me for months. Her voice cracked as she tried to explain. She said she never meant for it to hurt me. That she felt neglected. That she thought I would understand. I was stunned. How could she think this was okay? I told her she was my best friend and she betrayed me in the worst way. I asked her how she could do this behind my back. She started crying and apologized profusely. She begged for forgiveness. But I could not forgive her. Not after everything. I decided to cut her out of my life completely. Mark begged me to give him a chance to explain. I listened but told him I needed time. I was too hurt. The next day I discovered something that changed everything. I checked Mark's social media. I saw a message he sent to Lily after our fight. It said he was planning to leave me. That he was tired of pretending to be someone he was not. I could not believe my eyes. It turned out Mark had been cheating on me with Lily long before I found out. They had been planning to break up with me together. I felt like I had been played. Used. I wondered how I missed all the signs. Now I am left questioning everything. Was I naive to trust them? Or was I just blind? Should I have seen it coming? I am struggling to move past this betrayal. My heart feels shattered. So now I ask you. Was I wrong to confront them? Or was I justified in ending everything? I am still trying to understand if I am the one at fault or if I was just unlucky to have the worst friends and boyfriend in the world.