Ms Queen
What is the evidence that you have an unstable shoulder?, have you had a dislocation event ?
Were you to reply that the 'evidence' , is pain in your shoulder , then my reply would be that this is not evidence of instability.
My interest is sparked by your description suggesting altered patterns of recruitment of pectoral and lat dorsi muscles, with your attention drawn to their apparent failure to perform.
By and large muscles don't misfire or have performance deficits unless they are involved in the following.
Inhibition associated with a disclocation sequelae.
Inhibition ( palsy )moderated by brain and/or peripheral neuropathy.
Inhibition associated with a temporary inflammatory event of Wikipedia reference-linkfacet joint(s ) and their adjacent nerve roots .
The third in my list is the most common and is eminently fixable , provided the therapist is clued into the prospect of spinal joint hypomobility and it's common thread of nerve irritation/altered recruitment/pain.
Were you to suffer at the effect not of a post dislocation event, but of lower cervical/upper thoracic spinal joint protective behaviour, then no amount of exercise would restore you to a pain free state. This seems likely given you have had 8 months of exercise in an attempt to restore normal recruitment and "stability ".
Protective responses in the upper spine are a common and potentially long term phenomenon, which may give rise to the experiences of pain, weakness and altered sensations in your upper limb and shoulder. If your therapist was not skilled in the differential diagnosis and treatments for the above mentioned scenario , you could suffer for far longer than necessary.
I would seek the opinion of a trained and skillfull Physiotherapist, other than the one currently treating you, who is willing to explore the prospect of referred events. Good luck .