Astrology doesn't fix one's future - it only reduces the set of possibilities in a probabilistic manner. If an astrologer claims 100% success rate, he is cheating you (unless he is a really special spiritual person)! Even a stalwart like Prof. B.V. Raman (who is regarded as the modern day Varahamihira) had many failures (as he himself acknowledged).
Astrology is a probabilistic subject in nature. An astrologer can only say that event x is "very likely" and event y is "very unlikely" - he can never say "this will happen".
Astrology is a probabilistic study of natural tendencies. It is a probabilistic study of the "gravity towards destiny". Just as the gravity towards earth can be overcome, this "gravity towards destiny" can be overcome by several means ("will power" being one of those).
In other words, astrology is merely a system that enables one to guess the future, but with a higher success rate than what can be achieved without using it. The two words in bold letters here summarize the weakness and strength of astrology. Astrological predictions are, after all, "guesses"! But then, they are "better" guesses (from the point of view of long-run success rate)!
There is a sloka (poem) in Sanskrit that says that only Creator knows what is going to happen - others can only guess it!