You sound like a bad episode of Sex and the City.....
Just jump on the horse and you may or may not fall off, but as long as the horse doesn't do a jig on your head you'll be okay
There's nothing worse in this world than 'NO' and that's not so bad anyway. If you never ask her out then you will regret it for the rest of life and if you ever date someone again you will resent them for not being the insanely beautiful and insanely intelligent girl.
The moral of the story, just dial some digits or stop by her place and take it easy