"Understanding" is head knowledge and did nothing to help your frustration. Anger is a goad to figure out how to resolve frustrating issues ("the inability to change or achieve something") and lets you know you have a problem that needs resolving. You wanted to talk with your T and were unable to! Doesn't matter why. That is anger. You have to do something to resolve the frustration, either find another way or person to discuss your issues with or decide you are comfortable waiting until you can see your T but dismissing the frustration, just understanding it away doesn't really solve the problem, the frustration remains.

What if your T had not been able to show up for 5 sessions (instead of 3)? What are you going to think next time your T does this again? You're so understanding, why shouldn't she? See the problems "understanding" rather than "Gee, I was really frustrated and unhappy you had to cancel three times in a row, I hope you aren't going to have to do that anymore?" and starting a conversation about it.

"No, no, I understand, it's okay. . ." doesn't do anything for you or her besides let her think it's okay to skip your session (rather than someone else's who expresses their unhappiness at her missing). It's a polite, social lie, that has no place in therapy, where you are trying to recognize what you are feeling when you are feeling it.

You could have a breakthrough where you are comfortable with yourself so when you cannot see your T because of cancellations, that is truly okay. I enjoy going to the dentist and know most people do not and my dentist and I know that if he has other patients or problems while he's seeing me, I will literally wait "happily" while he deals with him; I am fine being last and "used" in that way! But until then, when you feel frustrated in some way; think of a 2 year old and what they do when they are frustrated? They have a temper tantrum That's anger. Frustration is a mild form or or precursor to, "Hey! This ain't what I want! It is not good for me" anger.