Hmmm....I have been in a similar situation. I like you stuck with my girlfriend at the time and broke all contact with the person I had similar feelings for. Though she was not dating a close friend of mine. Ultimately my relationship with my girlfriend ended. By then I had lost contact with the girl I had such a connection with as she had returned to switzerland in the mean time. Here I am 17 years later still wondering what happened to her and what might have been. I believe many today stick in relationships because they do not wish to hurt the person they are in a relationship with or are afraid of losing both their partner and the one they have feeling for and end up alone
If I could go back in time I would do things very differently with the benefit of the hindsight I now have. I think what I am saying is to have a long hard look at what it is that you really want. Is your current partner truely the one you are going to spend the rest of your life with. Or are you just afraid to upset your best friend and current boyfriend. Consider the fact that if you and Kurt got together and you never saw your current boyfriend and best friend again could you live with that of would you rather never see Kurt again if he split from your best friend and hit the road.
There are no easy answers for you here....Good luck.