If she isn't that important to you, tell her to take a hike. And be sure to mention that you didn't actually cheat on her, and what actually broke up your relationship was her paranoia and endless harping.
If she is really important to you, then explain the situation calmly (Ask her before you start talking to let you finish) and ask her to please drop the subject, because nothing happened. If she doesn't believe you, or she is still being a . .. .. .. .. . about it, (I apologize for my language, but the word is apt) then dump her. If she won't drop it now when you come completely clean and ask her nicely to after telling her nothing happened, then she never will. Honestly, do you want to marry her and hear her gripe about this crap for the next fifty years? If she doesn't believe you, then drop her like a *insert idiom of your choice here*.
If she is really important to you, then explain the situation calmly (Ask her before you start talking to let you finish) and ask her to please drop the subject, because nothing happened. If she doesn't believe you, or she is still being a . .. .. .. .. . about it, (I apologize for my language, but the word is apt) then dump her. If she won't drop it now when you come completely clean and ask her nicely to after telling her nothing happened, then she never will. Honestly, do you want to marry her and hear her gripe about this crap for the next fifty years? If she doesn't believe you, then drop her like a *insert idiom of your choice here*.