I want my boss's daughter soooo badly but I have a girlfriend that I love.?

Before this I've never cheated a day in my life. I've dated my girlfriend for 3 years and I love her and known her for years. She's perfect in my eyes and I've always loved her (as a friend and girlfriend). However over the past few weeks my boss's daughter has been working my office (and by my boss I mean the head CEO of the company). When I first saw her, yeah I thought she was beautiful but I didn't think anything past that. However over the past few weeks, we've been working together closely and she made a p at me. When it happened, I turned it down and she never approached it again. One day about a week and a half ago, my gf came to my work during lunch (and I was eating with the daughter) and after she saw the girl I was working with, she got really like mad at me and grilled the girl. And now my girlfriend got all untrustworthy of me and we got into a fight. So I did the typical male dumb thing: I had with my boss's daughter. Now for some reason my whole like thinking has changed and all I can see is how perfect she is but I still love my girlfriend and I want to be with her. I don't know what to do about it. I feel awful and guilty and everything. What should I do? I've never felt such an attraction to a girl before as my boss's daughter.