I hope your issue has been resolved, but I recently dealt with something similar and wanted to share with you how Pebber's advice got me better!
First, I literally woke up one day about 6 months ago with no feeling in my entire right arm and all the way down into my right index finger. The arm numbness would come and go, but the index finger was numb & "tingly" 24/7. After almost 1 month of suffering in silence, I finally talked to Pebber about this (we've been friends a long time and we communicate with each other almost daily). I was literally crying about my condition and was convinced that I would never be able to play the guitar again! The entire arm was often completely numb.
Anyway, the first thing Pebber said was to completely stop playing the guitar. Pebber then suggested I find a chiropractor who does kinesiology muscle testing and acupuncture. I found such a chiro, and after about 3 months of treatment, I was 100% better. It turns out that I had a bulged disc at the base of the right side of my neck that was putting pressure on my nerves that ran down my right arm, to the tip of my right index finger!
If you are still experiencing your issue, please stop playing everything and go get it checked out by a chiro who does kinesiology muscle testing and acupuncture. It doesn't matter if you are young & fit: I spent years in tae-kwon do and boxing, and always played sports & exercised. It has to do with posture and nerves. You said that you would practice "10-12 hours per day," and I used to do the same.
Thank you for sharing, and I really hope you are doing better. If not, TRUST ME,,,you will be fine, as long as you get it checked out!