Um... Maybe I'm reading it wrong or there's just not enough information, but if just "talking too much" is enough to make her uncomfortable enough for you to say you'll leave her alone for a while, I'm afraid writing a love letter to her will just make her feel so awkward around you even a friendship's likely to be out of the question. What exactly is your goal with this letter? From what you've said here, it looks to me like she's given you reasonably big signs that she's not interested, so I wouldn't get your hopes up too much... Unless there was more, or more context, to that?