I'm guessing what you mean by "full extension" is reaching back with a straight arm during the backswing. If I'm correct on that, I'd have to ask who advises you that this is what you need to do. If you look in the video library on this site, you'll see what the pros do and pretty much all of them do NOT get "full extension on the windup".
I suggest studying those for a bit and how they do it and post some video of yourself pitching. Then we can have a more detailed discussion.