I've never done fentynal but I went on methadone after being addicted to Oxys, heroin, dilaudid, hydro morphs (anything to keep me from being sick) I stayed on for a year and slowly tapered down, it's been 8days and I'm still in pain too. But I know from previous Experiance, wds usually start to stop after 2 weeks but it's different for everyone. Take vitamins, hot baths and just don't give in. You will get there.
I have taken fentanyl several times for kidney stones and always in the hospital. What I know about it is, it is 100 x's more powerful than morphine. how much were you taking and for how long? What I know from my own experience with pain meds, and this is long time heavy usage, is that everyone's symptoms subside at different times, its very hard to judge when you will feel better. Me personally, with hospital detox and 2 weeks of treatment, I still had problems after the 2 weeks. I am still on medications to help with the lingering effects of withdrawal, and to this day I still have sleeping problems, jitteryness, and occasional hot and cold flashes and I am at 62 days. The worst should be almost over, just know that it does take a considerable amount of time to really feel better.
I have no knowledge about fentanyl but I will make this current for more visibility. You will find a lot of support on here.