Left arm shoulder injury. Left shoulder?

I started going to the gym since almost 4 months. I was able to do all the exercises there. But when it came to lifting the dumbbells, the right arm was functioning normally, but the left arm wasn't able to lift the dumbbell after a few minutes of exercise. My trainer told me that i have a left shoulder injury which happened before i joined the gym. Since he has done a course in physiotherapy, he told me to do some shoulder exercises, but even then the problem hasn't resolved. Only because of this left shoulder injury, i am not able to perform dumbbell exercises with heavier weights. There is absolutely no issue to the right shoulder, it is able to lift the weights properly, but only because of the left shoulder, my overall workout is impacted a lot. Please let me know what i must do to treat my left shoulder? Thank you in advance.