My should hurts when I throw anything?

I'm 15 years old and my shoulder feels like a knife is stabbing and twisting when I throw a baseball. I have a pitching trainer and a mobility trainer that have been helping me. My mobility trainer says its a shortened pectoral muscle that causes it to hurt. They think it is years of throwing wrong is the reason why it is hurting now. And it has been hurting for about 2-3 years already. Please I need help!