Rule out physical pain, as GG said. Once you do that, if there are no issues, this is usually pretty simple to fix.
Walk up to her like you're about to mount. Use your hands on the reins to make her move forward, back, and side to side. Ask her to stand. When you do try to mount, if she moves, make her work. I've found the side to side to be most helpful with a horse that tries to back away or bolt forward. Be consistent. Every time she takes so much as a step, make her work. Stay calm, and be patient. If you get nervous or frustrated, it will be counterproductive.
The most stubborn horse I've worked with on this problem took three sessions and now stands calmly every time.
Another thing to consider is what you do once you're on. I make all my horses stand still until I ask them to move off. One mare, I'll stand there for three or four minutes before I ask her to move. She is so much better, though, that I don't have to do that as often.
Learning never stops