How to trust my boyfriend ?

one morning about a month ago, my boyfriend [19M] called me [19F] to see if it was ok for him to add back his old “friend”. He had told me about her in the past and just said that they would talk about random things and her family problems. i trusted him and so i said go ahead but his response peaked my interest; he was saying things like “are you sure??” “i don’t want to make you uncomfortable”. i started second guessing if i was remembering the story with her correctly so to clarify i asked “well did you guys used to talk?”, at first he said no but later he said that they were attracted to eachother at first but after the first few days of snapping they just turned into friends and that’s all they’ve been ever since. given all this, i didn’t see a problem and encouraged him to add her back.

i kind of forgot about it until last night, when she popped up while i was playing a game on my boyfriend’s phone. i had forgotten who she was and so i asked and clicked on her snap profile. i immediately saw that they had a lot of saved in chats including things like bikini pictures. i was so confused because he told me they never talked and he was saving her snaps in chat up until a few days before he asked me to be his girlfriend. i don’t think that he cheated but i know and he has admitted that he lied about their relationship because he “didn’t want me to worry” 🙄 like bruh.

as a stand alone situation i don’t think this would fire me up too bad but this is the third girl i have caught him lying about and we’ve only been dating for a little over 3 months. he did not cheat and i believe him about that, he’s willing to do anything to make me stay but i don’t feel comfortable asking him to delete snap or anything so i guess my question is how do i trust him moving forward? what would you do?