bodidley, with what information that you have gave so far I would have to guess that the problem is your riding(i.e. pulling on the horses face) or a physical problem with your horse(ie. teeth), either way a cavesson or 'mouth shutter' is only going to Band-Aid the problem not fix it.
The use of a cavesson or mouth shutter in western training should be adjusted loose enough that it doesn't keep the mouth tied shut and lets the horse relax the chin but when the mouth gapes the horse bumps into it encouraging him to keep his mouth closed.
Either way I suggest you find the root of the problem before using one even though you might have to use one for a short while to break the habit even after the problem has been resolved.
I DON'T LEAD 'EM AND FEED 'EM, I RIDE 'EM AND SLIDE 'EM.