An American Comedy movie in the 90s or 2000s where one of the male stars (the lead?) is obsessed about an old girlfriend who he (wrongly) believes cheated on him. Years later he is still obsessed about the betrayal that never happened when he encounters her again, possibly at a high school reunion.

I remember a funny scene where after a lot of deliberation he tells her that he forgives her, thinking he is being magnamious, and to his shock she reacts by screaming and starts to hit him.