Username, It's entirely possible that by now, you have already given her a letter. If so, I wonder how it went. If not, one thought I had was that you could, rather than give her an ultimatum, simply say that you will no longer tolerate her indecision and that you are going to move on with your life. Then avoid telling her what to do or what you're going to do specifically. Leave her guessing. Plus, this way, you won't box yourself into a corner.
Obviously, the letter can be longer than that, filled with emotion, but without clear guidelines of what you're going to do next. Michele
