
awww ffliwt
I'm so sorry you're in pain.

I've noticed I do exactly the same thing - just tonight I was thinking I might try and raise my shoulder rest a bit because it was sliding forward with the way I was gripping it. I was clenching my teeth and gripping my viola with my shoulder and chin far too tightly. After a while I started to relax because I was concentrating on something else. I am also constantly shifting my viola around on my shoulder to find a comfortable place to make all the reaches I need for my 4th finger.. but that tightens everything so much my vibrato suffers too.
I have ordered a flesch chinrest - the one that sits centrally over the tailpiece and when I go to the violin shop next month I'm going to try some different shoulder rests.... so you're ahead of me there on things to try.
I get spasms in my neck where I already have an old orthopaedic injury: one of my vertebrae has been rotated from impact in a car accident. Sometimes this presses on the nerve in my neck and is very painful. The tension from viola playing also causes a neuralgia-type pain up my neck and the side of my face too.
Anyway, the reason I'm telling you all this is because the next thing I am going to try is a physiotherapist who specializes in deep muscle massage. She treats my husband for his mountaineering injuries. I'm going to take her a copy of the x-ray of my neck and also my viola so I can show her where the tensions occur and she can both treat and advise.
It might be worth trying to find a sports physio as your next step. I'm expecting to see her a couple of times so she can release the tension with the massage and give me pointers for reducing the stress-build up in the first place. Like you, I need to STOP tightening up that shoulder!!
However, although I only play from 30mins to an hour a day I also suffer tension in my right arm and wrist from painting too. When I join the orchestra next month I will play for a couple of hours once a week too so I need to sort these things out.
Which music college are you going to? Why not try calling them now and asking if they know of anyone who deals with physical injuries in musicians - I'm sure they're familiar with the problems! You do need to get this sorted out now if you're looking at spending your life playing.
The other thing I do is a little light meditation based on relaxing muscles all over my body. The pain from my neck injury was so severe that I had to take heavy painkillers that made me very groggy. Most of the time I couldn't face being so groggy so I needed to fine existential ways of dealing with the pain... and meditation and relaxation really helped.
Good luck - let us know how you get on

Edited to say:
I don't know where you are but I found this site
http://www.musicianshealth.co.uk/services.htmland this
http://www.edphysio.com/physio.10.phpPerhaps there's something like this near you?