I ’21F’ feel like an option after my boyfriend ’21M’ almost got back with his ex

My boyfriend ’21M’ and I ’21 F’ have been dating for a while now, but something that happened early in our relationship has been haunting me more and more and I’m really struggling to process it. About three months into dating, while we were on a call, he suddenly got another call. He looked at the screen, paused, and told me it was his ex. His voice started shaking. Then, without much warning, he hung up on me and took her call. He didn’t call me back. I was left confused, anxious, and honestly a little scared. The next morning, he had a flight back to the city where we both study, and still no word from him. It wasn’t until he landed that he finally reached out. He told me he was sure about me, that he didn’t want to get back with her, and that he was committed to us. 

Over the next five months, we kept dating, but his ex kept coming up. He’d talk about memories with her, things they used to do, even stories I didn’t ask for. It stung every time, but I stayed. I wanted to believe him. Then, five months later, he accidentally let something slip and everything fell apart for me. He confessed that after that late-night phone call with her, he didn’t just talk her down. He met her in person even after his family advised him not to. They talked about breaking up with me so they could try again. She told him how much she had changed, how she wanted a fresh start. And he told her he would end things with me. She even had the nerve to ask why he hadn’t broken up with me already.

He says something happened at the end of their meeting he won’t say what but eventually he said no to her. Then, the very next day, he flew back and acted like nothing happened. And now I don’t know how to feel. I don’t know how to be okay. I feel like a backup plan. Like I was the safer choice not the first. Not the one. Like I was kept on standby just in case things with her didn’t work out. It’s been tearing me up inside. I feel small. I feel disposable. I feel like a default.

Why didn’t he tell me when it happened? Why did I have to find out months later, by accident? Why do I still feel like I’m standing in the shadow of someone who hurt him and who still got another chance before I even knew what was happening?

TL;DR I don’t know what to do. I don’t know how to move forward from this. Has anyone else been in a situation like this? How did you cope? I just… I feel really, really broken right now.