need opinions/thoughts about relationship issues!

Manüella

so i recently found out that my boyfriend of almost one and half years has been using fake profiles with a different name but his photos apps and sites such as kik, snapchat, meetme, ashleymadison, xmeets, and various others to masturbate with other women online. mind you, i have no problem with regular porn! i understand that masturbation is something natural and healthy to do. but i had voiced that i wanted our relationship to be 100% monogamous at the beginning, and in my opinion that isn’t completely monogamous. i caught him with hundreds of videos and pictures of girls on his phone and laptop, as well as videos and pictures of himself that i had never seen before. i threatened to break up with him when i first found out about it (august, shortly after our 1 year and 2 month mark) and he promised to deactivate the accounts and not use them again. but i found him with more stuff in the photo cloud of his laptop and on the kik app, newly accessed websites for meet up purposes. he claimed they all were from before and that the phone syncs weird with his laptop to show wrong dates. i don’t believe him. i have voiced to him that i am at my wits end and he has one strike left about lying to me and doing this and he hasn’t even deeply apologized. i’m scared that if i stay with him i will continue to get hurt over and over again, but i’m also scared that if i break it off with him, i won’t be able to find anyone better than this.. and that i’ll regret leaving him down the road. any tips? am i asking of too much for an 18 year old guy?