<human>: So around two months ago I started dating this girl J who is two years younger than I. After the 3rd or 4th date we had sex and both lost our virginity (not sure if this is important to her, but isn't really to me). 

However, the Sunday before NYE I fooled around with my best friend who I had feelings for for almost all of 2012. On the car trip to drop her home she broke into tears and admitted she had had feelings for me for most of 2012 and had tried making hints to me earlier in the year but I had never fallen for them until now - when I have a girlfriend. 

Now I am of the belief that I can never fully love my girlfriend, or anyone else for that matter until I have tried a relationship with my best friend and seen it succeed and fail. It is unfair for me to remain with my girlfriend while still having stronger feelings for my best friend. 

I want to leave my girlfriend, but I do not know how to let her down. She is definitely more keen on me than I am on her, plus if I leave her now for a girl I've known much longer it will look like I've just used her for sex. So how do I explain this all to her and make her as least upset/mad as possible?

Can you condense the text into three sentences or less?
<bot>: My best friend and I have feelings for each other but I have a girlfriend that I don't have the balls to leave; how do I do it?