As BenMort said there could be a number of muscles involved. As it's a long term problem there may be more than muscles involved and not all muscles need stretching if it's a postural problem. Some are short and not working correctly and some are "longer" and not working correctly. The muscles on the surface so to say may be trying to compensate for deeper postural "instability" of the neck and need some reprogramming. (the small muscles mentioned above) You don't say what you do for a living but there may be an ergonomics/posture in every day life issue that needs assessing and correcting by a physio for example. I suggest you go to a good physio soon now as it's been so long without improvement. Maybe reduce the amount and intensity of neck stretches you do in the meanwhile as well and see what hapens. Lots could be contraproductive.... keep us informed!