I think you write down the feelings, let yourself feel them, and then when you go to your next session you let your therapist know how you’re feeling, including that it’s directed at him.
Regardless of whether or not he deserves to be the target of your anger, what’s great about this is that he is a safe person who will be able to hold that anger and help you feel it and work through it. Even if he screwed up and that’s why you’re angry at him, he won’t take any of it personally. He will own any responsibility he may carry and be OK with the rest too. So… go for it, express it to him in whatever way you can and need to, and he will be solid for you and help you through the whole process. That’s when you get to session.
In the mean time, here’s what you want to be aware of and notice when you’re doing it and if at all possible, redirect it when you see it.
If you do not feed emotions, they don’t last too long. A book I read says ninety seconds. I think for me it’s more like ten minutes. The key is to not feed them. How do we do this?
First, welcome what you’re feeling as a kind of messenger. It’s telling you something, trying to point to some kind of wound. Thank this feeling for its service, let yourself feel it, notice where in your body you feel it and what it feels like to feel it. Treat it as a feeling (or set of feelings) you are having an experience with. As an exercise and way to drive this concept home into your mind, when you journal about it, begin by writing “I am having an experience with feelings x, y, and z.” And then you can move on to describe the experience. Do the same when posting online. You are not the feelings. You are having an experience with the feelings.
Next, the part to notice if it comes up and then redirect. Do not try to associate the feelings with any particular cause. Do not say, “I’m feeling x because b and c are happening.” It’s not about accuracy. It’s about not feeding the feelings you would rather they pass through quickly. When you give reasons for your feelings, start associating them with things going on in your life, it’s like you give them fuel to burn longer. So you want to still acknowledge and welcome your feelings as valid experiences, but you don’t want to give them fuel. That includes justifying them, trying to shut them down, and especially linking them with reasons. If you find yourself doing those things, note it, and redirect your thoughts back to the experience of the feelings themselves. In journaling or posting, definitely do not commit the association of feelings to people or events to writing. When you notice it happening, perhaps repeat the opening, “I am having an experience with feelings x, y, and z.” Move on to describing the experience as is helpful to you.
When you get to session, then it’s fine to talk about possible causes and associations because you’ll have your therapist there to help contain it all, and it seems for the most part he does a good job with that. But to make it through between now and then, best to simply live the experience of the feelings while not feeding them.