There is a lot of good advice on here, I'm not sure I'll be able to add any more but I will share this thought, If you are worried about the deer spooking or jumping the string, don't you have done everything right up to the point where that deer has come under your tree has not busted you up to that point. You have been successful to be that close to a whitetail deer and that is what bowhunting is all about; at least to me it is. The thing is though, you can not control that animal, as much as you will it along; to stay broadside, turn its head away, move that front leg a little forward you can try all you want to make those things happen but when it comes down to it; you can't. What you can control though is everything else that has already been posted here; you're breathing, if you are already at full draw the hard part is over; if the deer has not seen you yet you're golden, so deep breaths, bend at the waste, aim a little low, blur out the vision of the deers head or tail, all you should be seeing is a bright pin through a peep sight centered on the deers chest.

If and only if, after all these things have happened, the deer is in your shooting lane, you have drawn and the deer is still there, you have controlled your breathing and picked your spot, if after all that at the release the deer spooks and performs some matrix move on your arrow, it just proves that $hit does happen and the only thing you can do is try again. No need to get discouraged because it happens to all of us.