[Magellan]

Since you don't seem to mind "negotiating" this situation with her, why not put this idea to her:

"Look, this is obviously something we're both thinking about at the moment and the anticipation of what might happen is hindering our underlying relationship. Why can't we just take the pressure off by getting down to business, and then we can do our "thinking" (tell her to take however long she needs afterwards) with less anxiety about things. It's just sex, and we've been friends long enough that I'm sure we can get over it if things don't work out in a romantic sense, so how about it? There's no pressure, it's just a thought I had."

I realise that the picture I've painted isn't exactly how you feel, that YOU don't need to think about it, and that you're hung up on her. Still, assuming you don't get a romantic relationship with her (but you keep her as a friend) you would still have liked to have had sex with her, yeah? This way is low-pressure, non-physical, and sounds well within the boundaries she's set. Alter what I've written above to taste, but I'm sure you get my general idea.