I don't think emails are the best medium to express anger. In fact, I think it takes some effort to make neutral emails not sound like the sender is headed over with an ax after they hit send.

You have to really assess what your goal is. If it's to vent, just state that it's a vent, but try to close with a suggestion for resolution, otherwise there is a chance that you'll regret it and feel worse. If it's to make her feel as badly as you feel, there's a chance that will not be a winning strategy.

It really depends on your relationship with your therapist and what you want to get out of sending an angry email, or what you want period.