How do I remove this stubborn ceiling light fixture?
The light fixture should have come loose after loosening that nut.

Try the following:


Get a step ladder or ladder and place it directly bellow the light. 
Get on the ladder :)
Determine if the fixture is 2 separate pieces or 1 whole piece (glass + gold decorative ring)
Carefully grab the light fixture with both hands and firmly turn the light fixture a few degrees in either direction. Be careful as the glass from the light fixture should come free.


If it does not it could be stuck ceiling due to paint or grime as someone mentioned in the comments.

If this is the case, try being a little more aggressive in your rotation, again being careful as it can slide free.