Obviously you're making sure you don't dig her with your foot or make the mounting process uncomfortable for her in any other way?
Don't know whether this would apply to your mare, but it always pays to make sure a horse is standing square and ready to take your weight as you mount. One of my old farmer friends would always get hold of a young horse and push him back, and then give him a gentle shove to the side, just to make sure he was thinking about where his feet were before getting on. That way the horse was already bracing himself.
Also if a horse is young (and again I don't know whether this applies here) it helps if you try and mount with your weight as close as possible over the withers, which makes it easier for the horse to balance as you get on, and also puts less strain on the weaker part of his back.