Time and patience walk your hourse over use the same word eveytime stand,stay, stop if they walk away when you try to get on walk over push them back repeat word and it might help takes time but wored for me:)
My horse doesn't stand for mounting either. What I usually do is if I go to get on and he walks away. I get off and try to get on again. I repeat this until I can get on without him trying to rudely walk away. Once he stands for me to get on, I reward him with a treat. I've been doing this for about 3 months and I'm seeing huge improvement!
You need to spend a bit of your time to re-educate him. I usually do this exercise : when you want to mount just stand on the left side of your horse, ask him to stand still and wait for him to obey. When he stands still, feed him a treat under his neck so that he reaches his head to the right to receive the food. If your horse moves, whithhold the treat until he's still and as soon as he obeys you must deliver the food reward. Then the horse needs to assimilate that if he stands still, once you have mounted, you'll deliver the reward.
Once your horse understands you can gradually replace the treat by just rewarding him with your right hand towards his right shoulder.
You horse need to learn the mounting process as a comfortable and funny moment instead of assimilate it as a stressful event.
Good luck I'm sure it's gonna be ok ! :)
If you had someone to hold them from the ground that would be a big help, if not you'll need to take some time to teach them to stay still.
You could dedicate a day to training for mounting. Just getting on and off from both sides, and every time they go to move just bring them back to the right spot. It'll take a while but they'll get it eventually! Then you might want to do it once a week or month so they remember!