The difference between these two is that "Could you not forget her for a time?" is an actual sentence, whereas "could not you forget" is grammatically incorrect (though occasionally someone might say "couldn't you forget" something, but always as a contraction). Today, we rarely say "could you not" unless we add a "how" to the beginning of the sentence or make it into a negative request. Example: "How could you not forget her?" It means she is unforgettable. There is no way you could forget. Generally we would drop the "not" if making a suggestion or request: "Could you take out the trash," but not always: "Could you please not make that annoying noise?" But we wouldn't ever say "Could not you make that annoying noise."