Man, doctors are useless when it comes to things such as this, as they typically only superficially solve the problem by covering it up. What you need is a specialist, but even then, in my experience, it never goes away. There's lots you can do to reduce it, but it'll never go away.
I got caught up with a bad case of carpel tunnel syndrome and tendonitis by playing those hours with terrible technique, got it relatively cleared up by means of therapy and such, but then found I couldn't play 20 minutes without some lighter form of it coming back to haunt me (mine is like an electric shock shooting through a particular part of my wrist up to my elbow whenever too much strain or a particular movement is performed on a regular basis).
I had to change the way I played 3 times until I found a posture/hand position that was the least stressful on my wrists and now, 6 years later hahaha, I can go for a good 6 hours maybe 3 days in a row until it starts to build up again. But I just started a construction job which is labour intensive and the strain is apparent when I try to practice afterwards.
It's like an old buddy, my wrist pain, and in the long run has done more good than evil. You just have to be patient with it and find ways around it. Consider it a sign from above that you're to improve your posture and hand technique.
