I own a business that I started before I got married. How do I protect giving her assets?

As the old adage says, prevention is worth a pound of cure. Even before you become married, your first defense would be to have a prenuptial signed excluding that asset from any of the marital property. If you have not done that, then what you try to do is only fund that business with non-marital income if possible. Your third alternative then is to set up a distribution of marital assets so you could protect the business so that you would own it at the end of the divorce.