Personally I think a large part of it (just going off what your post is saying) is that somewhere in your mind you are hesitant to be with this girl. There is definately signs of you holding back. You feel at least to some degree that being with her (since she is in a long term relationship) is wrong. That is part of it I would think. Men as well as women can certainly put mental blocks up that have powerful effects on us.

If it isn't that or in addition to that I believe like the other two posters that shyness, maybe the sense of "I can't believe this is happening finally" etc are playing a part in it as well.

For me personally even with a woman that I love without problems or holding back I can be so sensitive from it that it takes a long time for me to cum through oral. But when I do it can be much more intense than a normal orgasm. I had an ex that had TMJ and couldn't go down on me for long but loved the taste and the feeling of me finishing in her mouth. We would make love and have fun and just as I got close naturally she would slide down and let me finish in her mouth. That way worked every time for me. The excitement of being in her body and the anticipation of finishing in her mouth worked for me.

My sugestion.. work first on the thoughts and feelings behind your current relationship with the lady. Believe it or not it has a big impact on this. Not just this but other things as well.

And if you continue to have the relationship with her try different things mixed in with it and coordinate so that when you finish it is with her going down on you. Not sure if this helps or not.... Good luck.