should I stay or leave?

My boyfriend and I have been dating for almost 4 years now and lately there's been plans of getting married and a lot of hints that he will be proposing some time this month. However, recently we hit a slight problem. While I was on his computer, he accidentally clicked on a bookmark of an image of a girl we both went to highschool with and then explained to me he had liked her way before we had gotten together and that he had been stalking her on social media throughout our entire relationship. He admitted that he knew one day I'd find out about it but didn't expect me to be all that hurt by it. Finding all of this info made me feel like I was never enough for him since he wasnt planning on stopping anytime soon before i had caught this and it makes me worry if he's hiding other things from me. I don't see him the same way I used to anymore. what should I do?