This is more than okay. It is OKAY to be angry at him. If you tend to always make him think you are fine in session, I would suggest writing a letter to him RIGHT NOW to capture your emotions AT THIS VERY MOMENT. It's really important because you might not be able to re-capture it at your session. If you write a letter now, and read it at your session, it may bring back some of the emotion, and that would be great. To get angry at him in session is to be on your own. It means that you are able to own your emotions in front of him. And that you don't feel as if you need to be "okay" just so he'll think your fine. If you allow yourself do deviate from that, you have taken a step towards autonomy.