I don't like how every time you send or receive a love letter from her you're adding conditions. Under this framework, that means you're changing your girlfriend! I'm not sure what the point of monogamy is if every time you correspond with your mate you switch to a new one.
I think the condition from the beginning is that you're picking a unique girlfriend who knows all microphysical facts about your universe, including the content of any letters you have or will ever write.
I just realized I haven't previously pointed the metaphysicians on Less Wrong to "Possible Girls," a hilarious paper about acausal romance:
Even if you don't read the whole thing, don't miss the final paragraph.