Men who are in happy sexual marriages treat their wives with love, care, kindness, tenderness and understanding. Another reason women withhold sex is to get something out of you. Also has the book of pook and other good stuff: If you go without for a little while, big deal, your dignity and self control and your future is worth more than a nut. That is so harsh to know that women will intentionally make a man suffer in that way while proclaiming they love em. But much more importantly: You hung yourself in this scenario.
Don't give her any attention, don't look at her or touch her while doing it.
If you have not given her a clear cut idea about what the future of the relationship is, she is liekly to wait and see if you commit. There are some specific "don'ts" to keep in mind, too: She should be begging you. If it's only an ltr then it's not a problem, you drop her like a bad habit and that's that. At that point, whatever you say can't hurt you.