Advice?

ive now been seeing a boy for 9 months now, from the start I’ve been loyal and respectful as that’s the way I’ve been brought up and have always been told if your not happy to leave. However everything was fine until about 4 months in when it came out the girl he was seeing before me he was still meeting and sleeping with behind my back, meeting her and then picking me up right after of course I ended things , he came crying to me etc saying he was sorry and didn’t mean it bla bla and being silly I forgave him however have never forgotten. We went through a stage of being really good for a couple of months after this and he was slowly gaining my trust back, it might sound silly but posting me on his snap chat etc everybody knew we were together . Now for the last 2 months he’s become very secretive again sleeping very close to his phone. Well last night I got hold of it , he always clears everything on his phone and a lot of people know him so I understand people message him but I guessed the password to his my eyes only on snapchat and found a video of a girl in her underwear which was recent talking last couple of days I haven’t mentioned anything yet and I know they say what’s the point with no trust etc but I just really don’t know what to do anymore , constantly feel like I’m in competition and it’s really starting to drain me , do I walk away is the question I suppose ?

Also to add I do live with this boy and spend every single night with him, I’m 19 and he’s 22