Astigmatism is just a particluar version of short or distance sight. It is can get worse or better throughout life. It is said that astigmatism get worse through the growth years up to about 22 years and will slowly declines until it shows an tendency to increase after about 60 years of age. Anyways, the human body is always degenerating. Even if you get Astigmatism, you can get comfortable, soft contact lens or fashionable eyeglasses to help you see things clear.
I am not sure whether the astigmatism will get worse or stabilize. But I believe it will slow down and slowly degenerate. I don't think you should worry about this problem. As long as you treat it in a proper way, it won't get worse. Untreated astigmatism will cause blurry vision or double vision, which may affect your vision if it is serious enough. However, there are some contact lenses that are specially made for astigmatism.