Definitely not your own.....
Are you romantically involved (or trying to be) with this one and its not working out? 3 years is a hell of an effort and it sounds like you've given it all you've got. May well be time to walk away, or at least withdraw your attention, if you can bring yourself to do that. They may well realise what they are losing and it could be the kick in the pants they need to change their behaviour.
The letter my not be such a bad idea. If she has it in writing she can come back to it a re-read it multiple times, rather than just hearing it once and forgetting it.