Fentanyl can be very difficult to come off. The withdrawals seem to be harsher and last longer than other opioids. But you are lucky to only be at 12 mg over 4 weeks. Did your doctor have you on something else before he put you on the Fentanyl? I ask that because Fentanyl isn't normally used as a first-line drug for pain. If you were on something else, that would factor into your withdrawal.
You can read lots of withdrawal hints over on the Substance Abuse forum. Even though you're not addicted, the physical dependency is the same. They have lots of supplements and other things to suggest. Staying hydrated is really important and sometimes it can be hard to hydrate if you're having trouble keeping drinks or foods down.
Personally I would never touch Fentanyl again unless I knew I'd never come off it. It was not a good drug for me and really wreaked havoc with my body. There are people who've had good luck with it but the side effects weren't worth it for me.
I am also surprised that your Doctor put you on the patch before giving you something in the Oxy family. This isn't normal practice. I really can't understand why he did this to you. Fentanyl should be for extreme cases only... Like cancer for instance.
It's Doctors like these that have people so addicted. I would consider finding a new Doctor. Seriously.
I'm in agreement with Mollyrae as to her suggestion about seeking out a new doctor. Dealing with chronic pain can be difficult. I also agree with her about Fentanyl only being used for extreme cases like cancer. Finding a good pain doctor can be difficult ... it seems there are doctors who write prescriptions for narcotics for a hangnail and doctors who are afraid to write a script for anything. Do some research and try to find a doctor who will manage your pain properly. Pain management isn't no pain and it usually isn't only medication. Depending on what your CP problems are there may be procedures, OT, PT, ice, heat, etc. that may help in conjunction with other things like medications.
In defense of my GP I am extremely sensitive to a whole bunch of meds and Fentanyl was a last resort. I can not take any form of NSAID's and any neuropathic treatment such as anticonvulsant or antidepressant sedated me to such an extreme it was intolerable even at paediatric doses. My issue is with the drug company who stated that stopping the patches could make me unwell? These is a huge difference between being unwell and requiring antipsychotic medication and I would never have even considered them if I was in possession of this information.