I suffer from fatigue too and for me, it is one of the worst things (along with the pain) about having arthritis. The inflammation process itself seems to cause fatigue, as well as the medications.
It's hard to have to slow down when you're used to living life to the full (I'm only 40), but I've learnt the hard way to pace myself. Resting and napping seem to be necessary to many of us!
All the best,