Why cant i forgive my bf for something that happened long ago?

i have been in a relationship with my bf for about two years now and we have even started talking about marriage and stuff. however my problem is that i just cant seem to forget something he did to me a long time ago. like all couples, we have had our share of fights but we had one major one when i found out he lied to me about going out with another girl. it wasnt a big deal or anything but i just couldnt take it that he lied straight to my face. i forgived him and he changed a lot for me. and i just forgot about the whole incident. the problem is, after about a year, for no apparent reason, i just recalled back about this incident and i cant find myself to forget it. in fact, i am at the brink of ending things with him. which is unfair, because he had changed a lot for me. why do i feel this way? what do u think should i do?