I have sent my therapist many frustrated mails. I wrote that I hated her, that she was bad at her job, that she was selfish, incompetent. It's what I felt at the time. I get angry very easily and she had hurt me (not on purpose). My therapist has never reacted badly, instead she pointed out that I was angry and told me we should explore that anger. She didn't take my outbursts personally. I don't know how your therapist will react and I don't know your relationship with her but imo you should definitely send that email. Even if it's venting, it's good for her to know all about it.