obsessed with my boyfriend ex

I’m 18 and my boyfriend is 21. This is my first relationship & he has had a few long term relationships in the past. He genuinely is a great boyfriend and takes really good care of me, but one day I got this really strong feeling to check his phone. When I did, his ex was texting him photos of them when they were together, some of them were of them in more ~intimate moments~. My boyfriend did not text her back at all but she was texting him almost every hour for DAYS. When i asked why she was unblocked in the first place, he said he checked up on her about a month ago because she was in a bad place mentally and struggling with drugs. Ever since i’ve seen these pictures i’ve become extremely jealous. She’s so beautiful, not a single flaw. I keep comparing myself to her, looking at her social media accounts every day and just tearing myself apart. I am so scared he’s going to leave me for her because of how gorgeous she is and how much they’ve been through in the past together. He reassures me everyday that he would never leave me for anyone but this insecurity is ruining our relationship. I am logged into his *** & made him keep his location on at all times. I’ve been trying to be a better girlfriend than her, i’ve bought him flowers and write him letters whenever I can. They broke up in march but every few months he finds his way back to her, even if it’s just a few words being exchanged. He promises he’s 100% over her but i just can’t believe him. I don’t know why he would chose me over her. How do I leave his past in the past? How do I stop ruining my relationship with my boyfriend over my own insecurities?