This is an old problem, which has affected a good many archers over the years.
A couple of good sources to try include the book "Instinctive Archery Insights" by Jay Kidwell, and the posts on this forum by Joel Turner under the caption "Target Panic Reality Check," and a related DVD.
Different things seem to work for different people. Something seems to work for everyone. Nothing seems to work forever. My own personal experience is that it can get pretty bad. Once you conquer it, the best way to keep it from getting really bad again is to nip it in the bud as soon as it starts creeping back in the door. For example, if your problem is that you release the arrow prematurely, and you EVER release the arrow prematurely again, for whatever reason that might not even have anything to do with target panic, make sure that the next five arrows you shoot are not released prematurely, even if you have to forget about aiming and all the rest of your form to do it, even it you're in the middle of a tournament when it happens, or even if you just released prematurely on the biggest buck you ever saw. Quit hunting, quit shooting the tournament if that's what you have to do, until you shoot five arrows without releasing early. You're not going to enjoy the hunting or the tournament if you're releasing early anyway, and the one thing I can guarantee you is that it's not going to get any better until you do something about it.
But first you have to solve your immediate problem, and Jay Kidwell or Joel Turner would be good places to start.