I'm so glad to see you've joined us as a member, Mr Dime - welcome to Mudcat! I can't say anything that hasn't already been said above, but one thing bears repeating, even shouting:

Do not, DO NOT, DO NOT "PLAY THROUGH THE PAIN". Not with tendons. EVER.

Your condition has been caused by wear. The only thing that will help is to refrain from doing the thing that is causing (i.e. aggravating) the wear, and give the affected areas TIME to heal. You need to ease yourself back into it, but only when your body is ready to. And please alter your wrist position! I'm a harpist and pianist, not a doctor or a guitarist, but I know that with harp and keyboard technique (and a classical violinist in the music school where I teach said a similar thing applies to her instrument) you keep the wrist AS LEVEL AND STRAIGHT AS POSSIBLE.

I dabble a bit in guitar and bought some classical-technique books, and all the how-to-do-it-properly pictures & diagrams show the fingerboard being held at a higher, more oblique angle to the floor (far less parallel, instrument propped on the raised left knee) and the thumb NOT wrapped around the fingerboard but supporting the back of it - allowing the wrist to remain as straight as it can. (See blue link below)

When you use fingers with a severely bent wrist, you are making the tendons pass over a constrained stress point which is causing them to wear against the casing. It's no co-incidence that all the instrumental techniques I know of emphasise the level wrist as an important part of playing position. You have to adjust the way you hold your instrument - and also sitting height - to give your arms maximum freedom and relaxation. (Computer use is an awful RSI-culprit too, and again one reason is wrist angle. Re-examine how you use its keyboard, in case you are unwittingly aggravating this problem, and change your method of typing if need be - there are by now a lot of computer-ergonomics websites and books out there. Sitting at a qwerty keyboard isn't that different from sitting at a piano keyboard - again lower arm and wrist must stay pretty level.) You can see from all this that, basically, anything that bends you into an angle is going to give your body more wear (aggravated by moving the individual fingers) and the more extreme the angle the worse it gets.

Putting on my music-teacher hat for one last time: Don't practice for 8-hour stretches. It's just too much. A lot of students are having to be told this these days, as competition and the demands of the pro world push the standards higher and higher, because over-practice is actually counter-productive. Also, it's too big a chunk of your day, which then gets deprived of other mental stimulation and physical exercise. (I sound just like my mother. But it's true.)

In addition, though it seems a bit fanciful & new-agey, you can help keep in trim with mental practicing. Not just daydream images, but actually go through all the playing motions in your mind, imagining every individual movement in clear tactile detail - i.e. how it feels - as though you're actually playing. This visualisation run-through is more effective than you might think, and there is some scientific evidence for it, along the lines of how the brain-patterns relate to and impact upon the muscles, with some very real physical manifestations (for instance, that old example of telling a hypnotised person you are about to drop a red-hot coin on the back of their hand, actually dropping a cold one, and watching the skin redden & blister - not a pleasant little story, but it speaks volumes about the mind-body relationship). I can't explain it better than that, but there's plenty of info on this subject if you research it. These techniques are known in the sports world too, and a lot of top athletes do mental work-outs. Don't worry too much about muscles atrophying - it doesn't happen that fast, and you can always keep them moving in non-painful ways. If you've developed a strong finger technique on your instrument, you'll be able to get it back more quickly than you might imagine - it provides a strong foundation for playing, which is why music teachers make such a fuss about it.

If you've already experienced RSI to this degree, it may flare up from time to time so you have to remember NOT to try and fight with it. Make whatever physical adjustments you can to your playing/working habits and try whatever anti-inflammatories & treatment your SPECIALIST suggests. (GP's & general physios are often just not clued up enough in this area: I've been horrified at seeing students here - not mine! - told to "exercise" when what they should be doing is the the exact opposite.)

Keep your spirits up, and develop ways of dealing with flare-ups as they periodically occur, because they may (and I think your current one has been made worse by what I can only see as bad advice). It will probably be a matter of management rather than outright cure. And whatever you do, DON'T jump straight in at the deep end the minute you feel better. Ease in, build it up step by step, and give yourself time. With intelligent care and a few alterations of habit, and the fact that you are young, there are plenty of grounds to be optimistic.   

Very best of luck to you, good wishes for the new year, and be sure to let us know how you get on.

http://www.tuneup.com.au/biomx/wrist.html