Yikes! I said something similar to my T once. It's a rare occasion that I've felt that way about anybody.

Your T is special.

This is what I would tell you (as a friend). Continue writing the letter with every intention of giving it to your T "someday". Download it from your computer to a disk, then delete from your hard drive. Store in a safe place. Tell your T that you are working on a letter concerning your history and hope to be ready to give it to her "someday". You don't sound ready just yet (IDK) and something this important, I think, should wait until you are really ready to tell her. I don't want you to have any regrets about giving her the letter. This is a BIG step. If T is as great as you say she is, she will be pleased and congratulate you on writing the letter period. This is MAJOR progress for you. Personally, I think it says a lot about the effect your T has had on you. I think one day you will be ready to either give it to her or read it to her (email is too impersonal for something like this). May I say that I admire you for having written the letter. Whether or not you give it to your T, it will help you.