You don't have to pretend that you just met.

I don't know what type of visa you have been using. If it is a tourist visa, getting divorced and married in the U.S. could raise the specter that you misrepresented your intentions when entering the U.S. (I don't believe that just getting divorced would raise the same questions.) I would suggest consulting an immigration lawyer with the specifics of your case, and determining if (after divorce) you should try to enter on a fiancée visa in order to marry.