What you need to understand is that paying your grandmother back does NOT mean the crime goes away. The crime was complete when you wrote out the forged checks and used them. When your grandmother made the criminal complaint, the decision about what to about the complaint rests completely with the police and prosecutor. They do not have to drop the case just because the victim asks them to drop it. Crimes are not prosecuted to benefit the victim. They are prosecuted to benefit society. By prosecuting you, the purpose would be to punish you for your wrongful behavior, with the idea that you won't do it again because you won’t want to face that punishment again. Also, it may deter others from doing the same thing. Paying her back may at least reduce the sentence you receive if you are convicted of the crime, however.
You need to talk to a criminal defense attorney to find out what you may be able to do to minimize the impact of this for you. But don’t get your hopes up too much. Folks don’t have a lot of sympathy for someone who steals from their own grandmother.