Sorry to hear about your MS. That dwarfs my problem.
There were a number of factors contributing to what I'm pretty sure was TOS. The sad thing is that if I had been diagnosed with TOS (which was known at the time I went to seek medical help) I could have saved about 15 years before I discovered my problem.
But after becoming aware that I had TOS it only took me four weeks to almost get completely healthy.
That's the problem with many mechanical/postural problems. Even when you correct your posture or some other wrong thing you're doing with body, it often takes weeks/months to see the benefits.
But I was very glad to finally know what my problem was. I wish I had started a program of stretching, use of a shoulder/back brace, and sleeping on the unaffected side when I figured out that I had TOS.
I wish you good luck in getting help for your problem. I'm just a petty whiner compared to what you're going through.
But your teachers and parents were right.....sit up straight, walk upright, and don't slouch. Good posture is every important for good health.