Hurts to "press" and "pull"

[Korch]

So my problem with my shoulder extends now to two exercises that have always been incomfortable (un-natural feeling) for me. The presses (like military press) and the pull downs on a bar that go behind my back.

One of the first times in my gym a trainer gave me some advice b/c. He said you've really got to build up your back, it looks like your back muscles are crap and your chest (benching, etc.) looks tight and it messes with your posture. You've got to have some balance to be fit.

Perhaps this imbalance is what's causing me some trouble with my presses. Unfortunately, I need to take it easy on the presses and take it slow (so I don't injure myself). All the while i'm still doing ballz-out work on the other excercises I was doing before. How am I going to be able to correct the imbalance when I have to do really small weights on my weak muscles (for fear of injury, not b/c they're simply weaker)?