I just read all the above comments, and I have to disagree, especially with blue eyed pony. I think she'll make a fine horse for any western discipline, fast ones since she does like to go faster. I can see how she'd be good at barrels and such at local levels, and although I have to agree that she may not go to the higher levels in reining, I think she's a fine representative of a working quarter horse.
The problem isn't with her conformation as much as it is with her weight and lack of muscle. Once you get her in shape, she won't look half as weakly built. Especially her back end- merely lacking in muscle and weight.