I had a very similar light, and the key was this:
Any upward pressure exerted from grabbing the glass created friction that made it not turn.
You want to touch as lightly as you can, near the edges, and try to apply rotational energy only, with no pushing up.
Unfortunately, that is roughly impossible to do if you're reaching so high that you need to press up on the glass to maintain balance. So, you still need to solve the problem of getting higher (and you should NOT climb on anything placed on a bed unless it's highly stable). But if you can get a bit closer, try less pressure, rather than more.
It feels very counter intuitive, but it helped me in a similar circumstance.
One other thing that may help, suggested by none other than @Joel Spolsky, was to try using rubber or latex gloves to allow you to produce more friction without more pressure.
What you have is power to the ceiling, and 3-Way switches.
My best guess, is to wire the two blacks together in the ceiling, with a wire nut.
One of the whites should be marked. This will go to the Black of the fixture. The other white wire should go to the white of the fixture. And then connect the ground.
So the path will be Panel Hot -> Black wire -> [ Switch Common -> Switch Travellers ->Switch common -> ] WHITE -> Light -> Panel Neutral.
the stuff in Brackets [] is in the wall, and you won't see it.
See this http://diy.blogoverflow.com/2012/01/poles-and-throws/ for more information on 3-way switches.
Best Answer
Pull down hard and then turn the nut (perhaps use locking pliers)
what has probably happened is that a different nut at the top has loosened and is spinning freely along with the shaft, while the bottom nut has frozen up.
pulling down will increase friction with the nut and hopefully allow it to unscrew.