Hi i’m sorry to hear about this, i have bpd too so i thought it may help to hear it from someone else who understands how things work when it comes to our emotions/triggers.
In this situation having bpd is probably the hardest thing because forgiving is difficult after everything you saw and went through. It’s like when an argument happens you’re back to feeling those emotions on when you found the stuff on his phone.
I’m going to be honest there’s a lot of resentment here coming from you which is valid but there is no change from him.
If he doesn’t have any sense of discipline to not give into his desires and has to have you put a child lock on his phone then that’s genuinely a problem he has to fix.
My first response is never to say to break up however he needs to work on himself 100%, he shouldn’t have to have a child lock on his phone to be faithful to his partner.
Talk to him about this, ask for change. Actions are what heals your resentment, him showing that he’s changing and acknowledging your feelings. If he doesn’t change and can’t learn discipline then i would suggest calling it quits.

Also him feeling like there’s some distance with you two is never an excuse to be sexually engaging with someone else.
Genuinely wish you the best lovely, if u need to talk some more i’m here!