I would also say, don't expect a reply. However, I would send it, especially if your former T appreciated your feedback before. Of course, any good T wants feedback from an active patient, anyway.

You are not responsible for the T's feelings, so the fact that you feel you haven't progressed fast enough for that T to be happy with you should be of no concern.

If you include a sentence saying you would just like a brief reply regarding the receipt of the letter, that might be all that's necessary to allow the former T to respond to you. Some feel it is close to ethical guidelines to contact a former patient, especially one that is under another doctor's care currently. Don't take silence personally.

I'm glad you are continuing to work in therapy on your issue and wish you much success.
TC
Sky