how to regain trust after its been broken?

so my boyfriend and i recently got back together after not being together for two months. it was a bit hectic, we were still talking and acting like a couple but we were both entertaining other people. he was being cold towards me and ignored me sometimes and even went as far as hooking up with somebody. well i’m not necessarily mad at him for it because ofc we weren’t together but at the same time he was muting their conversations so i wouldn’t see it and lying abt his whereabouts. i forgave him (i guess thats what you’d call it) because he did say that i was doing the same except i didn’t hook up with anybody. but yeah we ended up getting back together because we agreed that it was either we cut eachother off and just do whatever we were doing or we work on us together. and neither of us really wanted to be with anybody else. but my thing is we’re back together but i just don’t trust him like how i used to :/ like before if he would take a while to reply to me i wouldn’t think much of it but now when he does and i see he’s on social media i automatically assume he’s facetiming another girl or lying to me again. and now i constantly feel like im not a priority to him even though i mean i guess i am but it just feels weird. i mean i would hope that he wouldn’t do something like that me because i think if he really did want to talk to other people and not be with me he would’ve done just that bc thats the type of person he is but i dont know what to believe now. my question is: how do you regain your trust for somebody after losing it? :/ i wanna make things work between us but i don’t want to be that girlfriend thats constantly on her boyfriend because she doesn’t trust him.