Should I, a 21-year-old female, sever all connections with him, a 24-year-old male?

Hello! I hope anyone reading this is having a wonderful day and is doing well. I was in a two-year relationship that felt perfect until I began to feel insecure about one of his friends, whom I'll refer to as Sierra. From the beginning, it was clear he had feelings for her in the past, but that didn’t bother me initially. I have friends who have transitioned from situationships to solid friendships, and I thought our connection would be the same. However, over time, I started to feel insecure about it. Fortunately, I managed to work on myself and those feelings faded. A month ago, he asked to end our relationship, stating he was feeling depressed and didn’t believe he could offer me the love I needed. While I didn't agree with that assessment, I accepted his decision because I can't force someone to be with me, and I understood how devastating it is to feel that way. We went no contact for a while until he reached out, and we agreed to take things slowly to see if we could rekindle our relationship. Despite this, I sensed something was off. I felt he was hiding things from me, and my insecurities about Sierra resurfaced due to certain comments and behaviors that related to her. I realize my actions were wrong, and I'm not trying to excuse them, but in a moment of overwhelming fear and resentment, I went through his phone. There, I discovered messages where he mentioned "keeping the door slightly open" for Sierra, and that he had only closed it because she had kissed someone else. He also said that even during our talking stage, up until a month before he asked me to be his girlfriend, he still had feelings for her. This revelation shattered me. These may seem like small matters, but right now, I can’t shake the feeling that I was merely a substitute; that he never truly moved on from her and was with me for the comfort of the relationship. I reflect on our two years together and wonder if it was all just a façade. If we had no significant issues, why couldn’t he let go of his feelings for her? How could he claim to be too depressed to engage with me yet find the energy to keep the door open for her? What hurts most is knowing they’ve spent time alone together, and it feels less like a friendly interaction and more like a rendezvous between two people who had unresolved feelings. I had confided in him about how insecure she made me feel, yet she is the one he seemingly pursued. I’m at a crossroads, torn between my thoughts and emotions, and it’s incredibly painful. I just need someone to tell me, “Enough, focus on yourself.” What infuriates me is that during our recent conversations, he has been so sweet and has shared beautiful sentiments, admitting that his main flaw is his lack of honesty with me. I genuinely love him and can't envision wanting anyone else at the moment, but I’m struggling to cope with the negativity surrounding this situation. Should I just cut all ties and walk away?