I am a fiddler & had big problems with tendonitis a few years ago. After heaps of tests, operations to remove ganglions and other largely ineffective treatment, I came across a sympathetic neurologist, also a musician, who suggested that the problem might be the way I held my wrist when playing. He suggested adopting a position that kept my wrist as straight as possible. I followed his advice also using warmth to reduce the inflammation The problem receded without my having to stop playing. Now at any suggestion of pain or discomfort, I have a good look at my posture and I usually find I have been letting my wrist bend again without noticing. I don't know if guitarists can make use of this idea but I can say that it was a miraculous success for me. Good luck...