I can see his texts

This is my first time ever making a post and hopefully my last. I (18 woman) recently found an old phone my ex (20 man) gave to me over 2 years ago while I was looking through books I never unpacked when I initially moved to where I am staying currently. When I found the phone and remembered it didn't work for some reason and decided to see if it would charge and turn on. It had no passcode to access so when I swiped to open it I could access everything. I wasn't entirely sure what to do and I sent him a text asking what I should do with it. (I know, it's very weird and very random that I texted him but I didn't want to just throw it away and I couldn't wipe it without more info). At first it had stated it was a completely different date and time to what it was currently, then it started updating. It wasn't too crazy, at first just time and date changing and things on the screen being updated until a message popped up at the top of the screen from someone I knew he used to work with. Let's call her Amy. The message had said I love you. Now, I'm completely over this guy and have no intention of ever talking, saying, or seeing this guy ever again but it brought back recalled conversations and thoughts reading this text from Amy and seeing his reciprocating response. I had been uncomfortable with their friendship during our relationship and at the same time was and wasn't surprised they were still in contact, let alone in a possible relationship. I felt it was better that I just drop it but then my name came up in their text thread after my ex and I had chatted about the phone situation. He said that I should just throw it out. They are no longer texting when I'm writing this but based on the little bit I could read from the pop up message I'm curious to see what they had said. (HE DOESNT KNOW I CAN SEE THEIR TEXTS). I do not want to invade their privacy but I want to know what he's saying about me. I know it's negative and I know it will hurt my feelings but I don't know if I can resist. What should I do?? This encounter has given me so much anxiety to the point I'm shaking.