I made a big mistake but is my girlfriend over reacting?

12 months ago I met an amazing girl on a weekend away through a mutual friend. She was intelligent, beautiful, sexy, fun and successful. Unfortunately she lived in a San Fran and I was in San Diego. (About 1hrs flight) We went out of our way to get together most weekends and spoke on the phone each night. I was besotted by her. Shorty after we met, we both committed to new long term roles in different States, she had signed a 2 year agreement which moved her to LA and her career meant a lot to her and she worked hard to get there and I have a business with many staff that I can't leave easily. I'm a fairly practical person, and as much as I was falling in love with her, I couldn't see how this could work. 2 years is a long time to not live together... especially as time wasn't on our side with her age being 39. At this stage we had spent 4 weekends together but spoke most nights on the phone. Although it was going well, she seemed to find lot of reasons to argue over minor issues and showed a lack of understanding of my opinions. It was rather one sided. I found myself a little unsure whether or not she liked me. 8 weeks after we had met, I went away on a business trip in Asia and then to a 4 day bachelor party in Vegas. On one of the nights whilst intoxicated, a girl in one of the clubs approached me and we kissed for about 30 seconds. There is no excuse for what I did. It was unacceptable. It wasn't something I have done before. I assume I did it because I was insecure about our relationship and had convinced myself she wouldn't care. When I returned back home and we caught up then went to an event the next night where I was in a great deal of trouble because I didn't tell her that one of my X girlfriends would be at the event in our section. I never found the right time to tell her... I never felt she would understand. I fell more in love with her over the next 6 months and believed that I would marry her. She always made it clear that she wanted to marry me. I finally told her about what happened in Vegas 9 mths later after an argument and we haven't spoken since. She claims that I Lied and Cheated on her for 9 months. I was the worst person that ever lived. I do feel terrible about it however I see it as a mistake and something that should be forgiven with time and understanding. I'm interested to hear what everybody thinks about this...