keep it simple. and pay homage to the deeply courageous thing she did by putting herself out there.
(1) express how deeply flattered you are and (2) simply say, no thank you.
that is all. leave it there. let it alone. even if she responds. say nothing, or if you must (and only if you must) reemphasize the above two points.
anything else that you might say--in hopes of making her feel better (or even yourself)--will inevitably be the thing she finds patronizing and upsetting. and of course, it will inevitably be the thing she replays again and again.
and then, and i can't emphasize this enough. in the days that follow: stop being so damn nice. stop being the guy she liked in the first place. it makes it that much harder.
she will like you all the more and all the less for the kindness you offer up. and she will feel crummy for not being able to meet your friendly gaze. so please don't ask or expect her to.
just a friendly piece of advice,