I usually fight through it and end up with a perfect group or two at the end but I agree that it is a mental game and the best thing to do is to walk away for maybe a half hour or an hour then come back to it. The more you practice the better you will get. Don't take this the wrong way, but you may not be that good, yet. Just keep practicing. Keep your bow holding arm steady and watch your grip. You should be like a stable cannon and the arrow should be like a missle launching.

Keep practicing!