I spent a lot of time with the horse before just stopping, I never just said, "Oh, she won't let me mount it's time to stop." I have kept on trying, but I couldn't make her do anything.
I'm a busy person, and with a lot of other tasks to complete I sometimes don't have time to stay there until I have successfully mounted. There are other horses on my priority list at the moment.
However, that is good advice to set time aside and really work with her. I'll make sure the next time I work with her I will have adequate time.