Sukerbut,
Your email to her was done with good intentions, a thank you and a clean break. I do have to agree with others here that subconsciously you were floating balloons and putting a noose around your neck for future issues. When you tell a former partner that they are "the one", you set yourself up as a liar when you do actually meet "the one". Women in general have great memories and she will certainly take a shot at you when you do find "the one". Telling her over and over that you lover her still and thanking her for changing you is bad. I know the purpose of this letter was to make you feel better but I would suggest in the future writing her a letter telling her everything about how you feel and how you hurt, then take the letter along with any and all pictures of her and burn them in a fireplace. The point is, she doesn't need to hear your words, you do. The sun will shine brighter for you than ever before as you move forward in your life and meet "the one".
Best of Luck!