Congrats!!!! That thing is a MONSTER!!! Sounds like a story you'll remember for a lifetime.
I've hunted deer and elk in the Rockies as long as I can remember. Heck I was concieved on a deer hunt but let's not get off topic. I've always wondered how run and gun hunting/shooting in the high country compared to military shooting. It sounds like they can be two different animals in some cases. I spend a lot of time and energy practicing for scenarios exactly as you found yourself in. I've even passed up some shots because I couldn't get steady enough.
You probably have forgotten more about shooting than I know. With that being said I have always believed that proper breathing was important to accuracy but when I'm out of breath I have found that if you can aquire your target quickly it's helpful to hold your breath and you will actually be a bit steadier. Works for me anyway. I only hold my breath though when I'm panting heavily......it's not something I would do in a calmer situation. Has anyone else tried this?
Try to put your tree stand in a tree with plenty of background cover, keep the prevailing winds for that time of the year to your face, and take care of those pesky squeaks and creaks your stand may have developed while sitting in the shed. A good treestand lube can be made by heating petroleum jelly until it reaches a liquid form. Some hunters have reported success by including a cover scent in this mixture before applying it to their stands.