Ithink jbruced's suggestions are good.
Evening up muscle strength and self correcting your posture are good, but I expect it's more than muscle imbalance, everything could be a little bit kinked in your spine.
I went to school with a keen violinist and she was already at the age of 15 having professional work done because of spine issues.I suggest seeing someone - I have found Mctimony chiropractice helpful in the past - it's not the push and crunch style, which I would be wary of. I only needed one session every year or so.
Also have you asked for help on violin fora? I bet there are some.