subbing: I have this same problem with my mare. I know I can never let her win, but after chasing her and circling her and etc for an hour I finally just mount her as she moving away, so she doesn't "win". Its aggrevating!
The key to something sticking is to have the time to fight till the last second of the day.
What I mean by "fight" isn't rough or physical.. it's a game. You are in it to win it. Your horse will never change its ways if you time out before the game has been won.
Meaning, you have to have ample patience. You cannot mount the horse when your horse can't stand. She wins, and she learned nothing.
I'm not drilling on you, I'm just trying to help you see that it'll only get worse if you don't teach her to stand.
If that means making her stand for 10 minutes, 15, 45, an hour without moving, undoing every move she makes (like if she steps forward 2 steps, sideways 5 steps and turns on her haunches, then make her turn on her haunches the other way, go sideways the other way 5 steps and back her up 2 steps.) When she stands, then you've won that battle. Release pressure by sending her off to walk, or asking her to walk on as that is what she wants.. but only on YOUR cue. Not a second earlier.
Yes it's very lengthy and boring and whatnot, but you get results. You just have to be a rock during the process, and learn when to release pressure and step away on a good note. If that means you don't ride, then try again tomorrow. Just get the result, and end on a good note.
Patience, consistency, and more patience.
Same to the OP. Win the battle. Don't give up because you're frustrated. If you don't have time to win, then don't even take your horse out to mount. Do something else like ground work or an in-hand trail ride or a nice grooming session.