Simple, you're not picking a spot and you need to aim low when shooting out of a treestand. I might also suggest that since you seem to be getting lots of opportunities, the next time a deer comes to your stand, just sit there and watch it for a while. Let your heart rate and excitement come up, then wait for it to subside. If the deer is still in a shootable position, pick a spot, aim at the lower part of the chest, then shoot it. The key being letting yourself calm down and making a good shot.