Never fails to amaze me how people will take a horse in and expect them to be perfect without allowing the horse time to adjust to the new surroundings, new people, new methods, etc. etc.... Of course, I'm assuming that this horse was moved when you bought her. I'm assuming that far more than a new owner changed in her life.
That said, why don't you just tack her up and lead her around yourself. Do ground work. Get to know her, her signals for when she's attentive, inattentive, frustrated, being naughty. Tack her up and mount, then dismount. Little steps. Do you have an enclosed area/arena you can ride in? When you're ready, then ride at a walk only in the arena. Move on from there... little steps... communication is the key. She will let you know when she is unhappy before the trouble starts, but you have to know how she is telling you.