I've written my T many times - I journal to her. It's different than what you are looking for though. For your situation, I think you could just call and leave a voicemail or send an email saying that you want to explore other options. Maybe you are just living out your avoidant pattern; however, from what I have read, I don't think this T is a good one for you. It feels like things are too blurry and you are on the receiving end of too much of her stuff.

Why don't you want to let her go? If you stand back and do the +/- of the situation, is her method one giving you more pluses than minuses? Are the minuses more damaging than the gains from the pluses? It seems to be that you should at least try a few others, because I don't think in the long run she is using a method that will work for you and I do agree with the general theory that the relationship is key if you are looking to resolve issues/traumas and such.