How to stop "pushing the ball"

When I pitch for some reason I tend to lean back, even though I start out with my chin tucked in a little (Like Roy Halladay). This causes me to throw side arm which will lead to an injury (it has before) Is there any drill I can do to correct this?

The only reason I can think of that you would sustain an arm injury as a result of throwing sidearm is that you’re doing something wrong. That delivery is actually the most natural, the easiest, on the arm and the shoulder, and you should never have sustained such an injury. Tell me—are you using the long-arm, full extension delivery (like Walter Johnson ujsed to do), or are you short-arming it? I used to use both, and the mechanics are a little different for each one.
To paraphrase an old saw: what’s sauce for the goose is not necessarily sauce for the gander. What will work for one pitcher may not be right for another one, and so what you have to do is find the stance that’s right for you. What’s right for Roy Halladay might well have screwed your pooch, and you need to get away from that.

I have been short arming it, my trainer is saying it is because I tend to lean to the left (I am a RHP) and it is casuing me to do that. I feel when I tuck my chin (kind of like Roy Halladay) I am able to fight the natural feeling of leaning, but it doesn’t always work (of course it’s goning to take some time to get right)