I don't have enough information to know why the police are prosecuting you and not the complainant. However, for whatever reason, the authorities believe him to be the victim, and you are going to have to fight the case, if you want to clear your name.
Sometimes, when there's an altercation between two people, the first one to the police becomes the complainant and the other once gets to be the defendant. Other times, the one with the more serious injuries becomes the complainant, and the winner of the argument becomes the defendant.
Other times, when the police actually come to the scene of an incident and they see the participants of the argument, they make a judgment call, and they simply find one person more credible than another. That person gets to be the complainant.
There are other ways as well that an incident such as this can come to the attention of the police, and can cause the police to arrest one person over the other. Without more information about how the police learned of this, answer that with any certainty.
What i can say is that in order to make an arrest, the police only need something called probable cause. Probable cause is just a reasonable belief that a crime may have been committed and that you may have committed it. The words of a complainant who was stabbed and who says that you stabbed him in a fight should be sufficient to give the police probable cause to arrest you.
To convict you, the state needs to prove its case against you beyond a reasonable doubt, which is a much more difficult thing for them to do. If he pulled a gun on you and you stabbed him because you had reason to believe he was going to use that gun on you, that would be self defense.
Self-defense is an affirmative defense. That is, it won't prevent you from being charged with the aggravated assault. However, after the state rests its case at trial, the defense would put on a self-defense case. If the jury believes that the complainant had the gun and that you acted in self-defense, you'd be acquitted of the charges.