I feel like a crazy person 😤

I don’t really know what I’m asking I just would like to know how others would feel about this?

My partner started a new job where he is away a lot I don’t think he would cheat as this kind of confirms, however he hasn’t had snapchat since before we got together and then only downloaded it when he started this new job. The girl he is messaging in This is an ex of his and honestly I’ve got no idea why he is messaging her. The only reason I went on his Snapchat is because it was always going off and he was getting funny about his phone, I’ve also looked and she is one of his “best friends” on there meaning he messaged with her a lot. I’m not sure what was said between them for him to say “I only sleep with my Mrs” but he obviously felt the need to delete that part which makes me feel uneasy. Now I know the best thing is to just talk to him about it but I don’t know how to even start the conversation and really would he be honest? Would it just be oh I don’t know she just messaged me and then we was talking like would it get turned around on me like I shouldn’t have been looking at his phone or what like ahh I’m just lost right now