I need some advice

My boyfriend and I have been dating for around 5 months now and it has been absolutely phenomenal. However, I feel like our relationship didn't start in a very good light. He had cheated on his girlfriend a few times in his past relationship and when we started to hang out, they were still together. He mentioned that he has thought about breaking up frequently because, for a year, they have been long distance. Eventually, he confided in me on what he has done because I told him that I also cheated once and we both seemed to regret it a lot. I was the only person he has ever told and I convinced him that it was terrible to keep it from her for so long and that he is wasting her time. Later that night, he told me that he told her and that they broke up. I helped him through his breakup and around a week later we ended up getting together. He said that his past relationship was like a painting on the wall, that it was nice while it was there but it is simply in the past. With hesitation, I agreed because he really felt like no one else has for me. However, I always struggle with comparing myself to his ex because everyone knows about their relationship and everybody probably thinks that I stole him from her. I have terrible insecurity when it comes to this and it kept eating me alive. One day I went through his phone, I'm not sure why, and I saw that he never deleted his photos of her. I confronted him about it because it made me upset. As a result, it only made him upset because he said that that was years worth of photos and never had the time to delete them(they were together 2 years). He deleted them after we had the conversation. Tonight I went through his phone, I was drifted to go through his exes messages. I knew I shouldn't have, but I wanted to know how they broke up because I felt like he wasn't over her. I saw messages of him begging for her to stay with him and how he wanted to have kids with her and live together. He told me before that he slightly thought of this with his exes but not nearly as much as he has with me. I'm scared that he never really wanted to leave her and I was just a rebound. I really don't want this to be true because we made plans to move in together after we graduate and our families are already so close.