My "mental excersise" consists of alot of "visualizing" success. I often replay in my head (mostly deer) coming into range, and what my reaction is. I mentally focus alot on the act of shooting and hitting the exact spot, and executinng the mechanics of a good shot. ie back tension, anchor, follow through. For me an important part is visualizing the arrow disappearing INTO the EXACT spot I have picked. Since doing this, when the time to shoot comes about, I actually think very little of what I'm actually doing.
A little story, since I've started my mental excersising:
2 years ago I made a shot on a HUGE buck for where I hunt, prob 140" deer. It was a steep downward angle of about 20-25 yards. My concentration, focus, and follow through was, in my mind absolutely perfect, the animal bounded off, and I watched, in absolute disbelief, the buck walk away unscathed. A close look with binos revealed an arrow in the dirt. Up close inspection revealed that my disbelief was FACT. I had missed!
The significance of this story to me anyway is that in my mind there was no way I could have missed this deer. It was an impossibility to my mind that the miss actually happened. I had visualized this scenario to the point where I was able to act it out with a cool, calm composure and execute in my mind, a shot that was perfect. I still struggle with this particular miss. Every aspect of the shot is still perfect to me. This is what I strive for on every hunting shot. Minus the miss of course!
Before this metal excersise routine, I would consistently fall apart at the shot. I could hardly get the bow up, I would see the deer, and let an arrow go "at deer" After starting this routine, all my good hits on game have been very well executed with the exception of one shot opportunity that happened real fast and had me shook up, being shook, and not focused, led to a poorly executed shot, and poor shot selection.
This is how I mentally train for the tough situations.
I also like to shoot a particular 3D couse that is set up to be quite difficult and closely simulates alot of real hunting situations.
Hope this helps!
Brian