6 months pp and I find messages 😭

Me and my boyfriend have been together for four years. He’s my high school sweetheart, we live together and we have a 6 month old daughter. He’s been acting differently these last couple days and I know it was an invasion of privacy but I looked at his messages and he secretly messaged a girl that works at a gas station close to where we live that he goes to frequently. He says all they did was flirt and she gave him her name and he just wanted to talk but I’m not sure I believe it. I don’t want to start over after four years and throw everything away but I want to make a good choice for me and my daughter. Please any advice on what to do and any trust exercises we can do or counseling to make our relationship stronger and not a complete waste?

Edit: in the messages he asked if she was at school and the second one was if she was at work both of these were times that he was either on break at work or off