One thing that I would add, sliding stops are not necessarily the best idea. While it may work for some people, the horse will anticipate the stop and will start slowing down too soon. My mare had some reining training put on her last year and she was amazing at sliding stops and loved to slide. Now that she doesn't have sliders and I am starting on barrels I don't like to run her to the fence to stop. She still loves to stop, but without sliders and often on different sorts of ground she just can't stop as well. She will pop out of the stop and it is harder on her legs to do that. I have stopped asking her to run to the fence and now let her turn a nice circle to the left.
It sounds like your mare is bracing herself because she is not comfortable doing a hard stop. You have to remember that it is totally different for a horse when they have sliders on. Plus with a barrel horse we want them to run hard till we are past the timer, which often doesn't give much time to stop before the fence. If you asked a professional rider and reining horse to stop at that speed they would do such a nice long slide that they would end up hitting the fence. It is unfair to ask your mare to stop that hard so fast.
Just my 2 cents.