Refusing to stand still while mounting is actually a somewhat common vice in horses. I'm glad you looked into any pain or discomfort first. But I think with most horses it's just a way of attempting to avoid work. I would suggest spending a little time every day working on standing still.
Backing up- horses actually don't love to back up. If she takes a few steps backwards, make her back up and keep her doing it until she doesn't want to back up anymore. (know her limits, some horses will get frustrated and start to rear) Then let her stop and walk a few steps forward and try to get back on. If she backs up again? Back her up again. You may have to have a friend carry a mounting block with you so you have one right where you need it.
Moving forward- this you will need a friend for. Have your friend put her on a lunge line and stop her at the edge of the circle (does that make sense?). Try to get on and if she moves forward back away and have your friend lunge her around a few circles. Stop, then try and get back on. If she moves forward, lunge her around some more. Over and over again until she realizes that walking forward on the line only creates more work for her.
Moving away from the block- I think this is the most common. This fortunately you can do yourself. When you get up to the block most horses will try to move away from you, but if you have a hold on the reins can only get their haunches away from you while their head is still at the block. So as you get to the block take the rein further from you (right rein, assuming you get on on the left) and pull it out and towards you so your horse's head/neck will bent and be tipped away from you (to the right). When her head is tipped right, she can't swing her haunches more to the right. You'll also need a hold of the left right so she can't just start walking forward. This take a bit of practice to get just right!
I'd spend several days just getting on, let her stand quietly for a few minutes, give her a pat, and hop off. Do it a few times in a row before you start your ride. Also, when she does stand still as you get on, make sure you give her a pat and make her stand still a few seconds before asking her to walk forward.
Sorry for the novel
but I hope it helps! Like I said, moving away from the mounting block is a fairly common problem but fortunately one that is usually fixed pretty easily.