Giant I have been in a similar situation and it is never nice.

My advice would be to ask yourself honestly, do you and this chick have a future? I mean look deep, you say she's really nice, are you just breaking up with her because you think you could find somebody "fitter"

If you still want to break up with her then just do it fast like a plaster. If the girl I was with didn't want to be with me I would want her to finish quickly so that I could move on and be happy elsewhere rather than dragging out a relationship going nowhere...