Should I stay with my boyfriend after he betrayed my trust?

My boyfriend and I have been dating for about a year and have recently become engaged. We are both 19 and in college. We attend college in the same state, but live in different states, so our relationship is long distance at times. About two months ago, he stayed over a friends house and ended up messing around with another girl who was also there. He kept it from me for two weeks before confessing. During that two week period, he maintained contact with the other girl, but claims she initiated it & that their contact was always platonic after it. After I found out, he severed their ties. He swears that he did not have with her, but I'm not sure I should believe that. I love him and I care about him, and have even forgiven his actions. It's just that I am unable to forget them. Since the incident, our communication has improved by leaps and bounds, but it is often still hard for me to feel like I can trust him. I know it will be hard, but I'm wondering should I move on?