I think knowing your motivations behind your actions is important.
1. did you write it just for you
2. did you write it because you were really hoping deep down he/she would call
3. All the above

In situations like this it's highly probable you'll be setting yourself up for failure if you're expecting something in return. In this case a phone call from your therapist. If you wrote the letter sticking to facts, and avoiding any kind of manipulation then I see no problem with what you did. You have the right to state how you feel. You don't have the right to a response. The specific dbt skill would be coping ahead, and you do it by preparing in advance what you'll do should you not get the desired outcome.

Cboxpalace, after I wrote the letter and was brushing my teeth before going to bed, it struck me that he could well not respond, and I did have a little talk with myself, trying to lower, if not eliminate, the expectation that he would call. I did not come up with a coping plan; admittedly I've got a severe deficit in coping skills. I decided I had to deliver the letter anyway and figure it out later. What happened later is what I wrote.

I do not know what I will do/how to cope if none of the three therapists I see this week don't turn out well. I just got home from the first one, and while she was very kind and compassionate, she thought, as I do, that I need someone who will return a phone call in between sessions. Not that I will call, because my trust is at zero right now, but I need that option to be available. Anyway, she's not that person, but she kindly made an offer to make a few calls on my behalf to find one that would be available in between and I took her up on it. I'm disappointed, and a little discouraged, but I'm OK for the moment. I feel kind of empty.

I thank the rest of you for your support as well. This is my only source, and I count on you.