I agree with Mark for the most part. When I had trouble with this, my sensei told me "just keep your breathing ahead of your activity" and that worked fine. It makes it work during "endurance" drills and in tournament fights as well.
Breathing is a 24/7 operation, and it will control the adrenaline rush if you learn to bring it up to the level of the activity you're engaging in. Your body will "learn" that you don't need the adrenaline to do your activity if you have your breathing right ahead of schedule. I don't think you can train to overcome the emotional side of it, if that's what's causing it, but you can try to sink those feelings into your subconscious and visualize how you're going to fight, rather than getting hyped up to go into the ring.