You know your horse, so I assume you would know if mounting were simply painful to her versus her just being sour.
If she is being sour, it depends on how stubborn she's willing to be. You said that getting off and making her step again doesn't work. That would be plan A and usually works. Plan A didn't work with my horse either; no about of resetting her position would do me any good.
First, make sure she knows the command "stand". When she moves, reset and say stand. Do this first on the ground, pretty much teaching her to ground tie. This may seen unnecessary, but it helps a lot. Make sure you know right were the line in the sand is. If she moves a single foot one step forwards, that's crossing the line. You have to be strict, because, more than likely, if you give her an inch she'll take a mile.
After she gets the jist of "stand", you can proceed in two ways. One, you can stand all day, resetting her position and making her stand. If she moves a foot, sharpy yell "NO!", reset her, and make her stand right where she was. Say "stand" and pet her.
The next approach is a little "harsher". Put her on a lunge line, halter underneath her bridle. Tell her to stand and move the mounting block next to her. The second she steps away, make her think she just pushed you over the edge. Yell at her and make her circle you on the line. She wanted to move, after all! After a lap or two, try again. Eventually, she'll get that standing completely still is a heck of a lot more fun than running circles and getting yelled at. Be sure to pet and praise when she stands correctly.
There are a lot of other ways to go about this, but that worked for me. I again encourage you to check her back and make sure she isn't sore before assuming she's sour!