How do I move past this?

My fiancé and I have been together going on 3 years and have never had any serious issues or arguments, this is actually the healthiest relationship I’ve ever been in. Until now…

A couple days ago I found a photo vault app on his phone and guessed the password (because men are simple creatures and use the same passwords for everything) and I found several (30+) nudes of girls who are not me. I could tell they were old as I knew some of the girls in the photos and know they were probably sent in high school. But there were also screenshots of a girl he used to mess around with and they were from her Facebook. The photos were zoomed in on her butt in tight jeans, bikini photos zoomed in but still with her face in them… it just made me feel so betrayed and gross. We have two children and our youngest is 5 months old. I’ve been struggling with postpartum depression/anxiety and self esteem issues and this just really took a hard hit on my mental health. I don’t know how to cope with these hurtful feelings and how to forgive him… he deleted it all and told me he hasn’t looked at them in forever but I feel like he’s lying because he had a separate folder with my nudes in them.. so he’s obviously been on that app recently. I am just mind blown that I actually found something like that in his phone. And when he apologized I could tell he really did feel bad and was sorry but I’m not ready to just say it’s okay. I’m so hurt and heart broken. I’ve had a knot in my throat and chest for 3 days now and it won’t go away. I can’t sleep, and now I have this thought in the back of my mind constantly that he might be cheating or something. I’m just so sad and I really need some help/advice on how to move past this. He does know I’m still upset but conveniently he had to go back to work (2hrs away, stays in a hotel) and he had to leave the same day it all happened so that made me feel even worse. If you read this far thank you and thank you in advance to anyone who offers me advice 💔