Absolutely, write her a letter. Whether you send it or not, just write it and then decide what you want to do. This will at least get your feelings out in front of you and addressing her at the same time. I think that is a superb idea. (I wish I had thought of it.)

I realize how devastating it would be to have your T pull away from you. But I think she can contain all of your feelings better than you can imagine. But you have to be ready for this. I guess I would ask you if you feel that you can go on without telling her. In my experience I was miserable, noticably miserable and I could no longer contian my feelings and fears and I burst into tears when I finally muttered my gut wrenching confession. BTW: She was not surprised.