It could be any of the things that you listed.You would need to post a video for us to tell if your form is off.Go over each exercise that works your shoulders and make sure your form is...
With proper training it is.Eat right and alot of it. Don't skip workouts.Don't give up and quit.Learn all you can about powerlifting.Work your deads and squats also as they will help your bench.
I think it is mostly up to you as to how far you can go by the time you are 20.You are your worst enemy.Work hard,Don't stop.Don't just think of how much you can lift by the time you are 20,also look...