I just found out...

So my SO and I went through a few rough patches of where he would add other girls on Snapchat or had asked for ones nudes. We decided to work super hard past that! 4 years later, I had been having a reoccurring nightmare past few nights recently that he had been cheating and I usually don’t do this but tonight I decided to sort of snoop in his phone! I’m on his Snapchat and I notice he has this girl blocked and I’m like hm? And so here I am it’s 2AM and I find out that this girl he blocked had applied to his store(he’s a phone store manager) and thats how they met, he was messaging her on messenger random “.” Periods like that and then would tell her he fell asleep holding his phone and dropped it so it made him message her, he eventually asked for her Snapchat which she declined but he goes out of his way to find her on snapchat. This all happened literally RIGHT before COVID and she has given me screenshots and I don’t even know what to think...we’ve been TTC for 3 months now and I feel lost. I don’t know if this is break up worthy, how I should feel, what I should do. I don’t want to lose him at all and he doesn’t want to lose me but why hide things like that.