Any narcotic should, if possible, be slowly tapered as physical withdrawal is quite unpleasant. I did a fast taper that was not supervised by my doctor
and it was less than ideal.
Fentanyl can be a bit harder than Percocet, Vicodin, etc. Drinking lots of water is essential as it is really easy to get dehydrated. You can taper from the Fentanyl down and then cross over to a different immediate release narcotic and then taper down from that.
Are you taking anything else?