shoulder impingment and scapular wall slides

I started weight training/bodybuilding just over 18 months ago for the last nine months I have suffered with pain in my right shoulder. I have seen two phsyio`s and both have diagnosed shoulder impingment caused by bad posture. I have been searching on the internet for additional exercise to help with this condition and I came across an exercise called `scapular wall slides` i have attempted to perform this exercise but find it impossible for me to get into the correct position. I cannot put my arms above my head touching the wall and have my bottom touching the wall at the same time, could anybody advise me on any additional exercises/stretches I could do to enable me to do this?