Help! I don’t know what to do!

My boyfriend and I have been together for 11 years, we are high school sweethearts! We’ve had our ups and downs but we’ve always stuck by each other’s side! We started dating in high school so of course we had the normal breakups or “breaks” as we liked to call it as kids; we were still young trying to figure out what we wanted to do! We even broke up for a while and did our own thing and we told each other if life brought us back together then we knew we was supposed to be together.. may sound cliche but it’s true and we’ve been together since. The reason why I’m posting this is because he’s been kind of distant lately. Not like super distant just off from his normal self. He just got a new job and I know that’s stressful but I’ve also caught him snap chatting a girl. I don’t care for him to have friends, I’m not controlling like that but I do not put up with any type of flirting coming from either end! I feel like if he even lets her flirt and he doesn’t say anything then it’s disrespectful to me! People have no respect for relationships now so of course I worry. I trust my boyfriend but I know he’s not telling me something! He’s showed me the snaps from the beginning and where she said she was “trying to chill” with him and he told her I wouldn’t like that. Of course I noticed her flirting a little but he kept making a point to mention me so I was like whatever. Well now, days later, I still see him randomly typing and when I try to look he turns the phone away or starts talking about something to like change the subject? We don’t keep our phones from each other, never have, we never hide anything from each other. This has been driving me crazy for days! He hasn’t showed me any thing from her in days and I know they are still snap chatting. He used to save the conversation and when I checked now nothing is saved. I don’t know how to bring it up because I know something is going on! I asked if she was still snapping him when I seen he had a message from her and he said she’s still just sending the stuff she uploads to her story! Please help me handle this! How could he throw away 11 years for flirting? I want to confront him and her so bad but I just don’t know how to do it! I can’t get screenshots and I know he’s lying straight to my face. My heart is literally shattered!