Weight Lifting Injury?

I am 16 and started lifting weights and would bench press 3 times a week and after two months my shoulders started hurting. I went to a doctor and don't have a rotary cuff tear bu he says to keep resting it. Problem with that is that it been 5 months and nothing has changed. The should joint doesn't hurt its just that the long vertical muscle strands that run the length of the should become irritated and painful when I use my shoulders to do anything. Any advice on what I have or any stretches or exercises that would help would be very appreciated.