Is flirting cheating when you are in a relationship?

Story time but going to make it short. So my boyfriend and I have been together for 8 months and still counting.... However, during the month of July (my birthday month) I felt something was off. So yes I did snoop his phone but since we both each have each other’s Face ID on our phones didn’t think it mattered since I trusted him and he trusted me. I was taking regular selfies on his snap when a group message popped up (the boys) you know... lol but imy thumb accidentally clicked it and went back to messages. BUT I seen a message of him and his girl bestfriend. ( yes ladies and gents i trusted him even with this female) smh 🤦🏻‍♀️ I saw there messages and well he flirted with her luckily it didn’t go further than that. But he never confronted about it. So instead , I would ask him like “hey is there anything you want to talk aboit or say?” He always replied with a no. So instead of reacting crazy I honestly took it upon myself to figure myself out real quick. He has to head home to get ready for my birthday dinner later that day.... yes I kept it within and kept it casual as if we were okay. Until it was time to drop him off. I asked again and again and NOTHING. So yeah I broke down. And told him and when he asked me to check his phone everything was deleted but I already had saw. 🥺🥺🥺 then it was midnight and it was happy birthday to me 💔😔and it didn’t go as planned. I guess what I’m asking is little advice ? Stay or leave? Will I be able to trust him again? It still hurts to this day but he’s been trying a lot . But can you blame me for not being able to trust him the way I use to?😔thought i have him everything he needed and looked for.