My girlfriend caught me looking up another girl's skirt?

When I was in Rome, I went to visit the ruins in the Roman Forum. Only my parents and me and my girlfriend went, my sister was in Germany with her friends. There was this stunning girl. She was breathtakingly beautiful she was sitting on this big rock and was wearing this short skirt. She had her legs apart and raised up, so you could clearly see her underwear. I was so hypnotized by it, my girlfriend was asking me questions, which I didn't even hear her speaking to me. She then realized what I was looking at and got really really upset about it. She slapped me across the face, told the girl I was looking at her underwear and asked me to explain myself. I just didn't know what to say apart from "i'm really sorry". Then later on that evening when we went for dinner I said "if it makes you feel any better I was thinking about you" but I think that made things worse. I feel really bad for looking up that other girls skirt but I can't and couldn't help it. She was wearing this white tight underwear I just couldn't take my eyes off. Even my dad was looking at it!, and my mom didn't say anything. I guess that's why my parent's relationship is a lot more successful than mine since my mom is a little more understandable. But anyway how can I make it up to her? I'm back from Rome now and my girlfriend still hates me. We have been together for a long time and it just hurts for her to get mad about something like this seeing as we have a lot of history together. It's not like I'm cheating on her.