Shoulder balancing or stabilisation issue during bench press

I'm experiencing a curious shoulder pain issue when trying to do bench pressing movements (dumbells or barbells). The pain is often excruciating, and prevents me from holding a barbell throughout a pressing movement or in an arm extended (balancing, stable) position. The pain seems to generate throughout the entire deltoid muscle and through the tendon running up (or down ) my left arm.

I tried lifting the barbell without attached weights yesterday, and had to quickly put the light barbell back on the rack because of the pain and the sense that my left arm (or delt) would simply give out regardless.

Sigificantly, I'm able to do many other exercises without pain or issues, including shoulders, back and arms. I am also able to do bench pressing movements without pain or issues when I use a Smith machine instead (although I've lost a lot of strength and size in the interim by relying on a machine instead of free weights) .

My reliance on the Smith machine and/or inability to use free weights for bench pressing movements suggests a stabilisation or balancing issue. Since the Smith machine stablises and/or balances the attached barbell and weights, I'm able to bench press to my hearts content (with only the occassional twinge or slight pain).

I have been doing a lot of stretching movements for shoulder impingements to little avial (via Bob and Brad videos) and have taken a couple of months off the gym - the shoulder just seems to be getting progressively worse regardless. It's been app 2 years now. Cortizone injections provide very temporary relief, and anti inflammatory medications do nothing at all.

What do you think might be going on?

thanks