I found my boyfriend talking to his ex-girlfriend

About a month ago, I went through my boyfriend's phone and found him messaging his ex-girlfriend that he was dating before me. He gave her a promise ring when they were together, and he still had it hiding in his memory box in our son's room. I found many things on his phone, but that broke my heart the most. He deleted all the messages but two. I can't help thinking they were talking about the old times together or something like that. It really kills me, and I can't stop thinking about it.