The journal you had before sounds like a good idea. You could also sit down now and pinpoint any thoughts you can recall you might have had when the incident with your therapist occurred. If you can't think of any thoughts, think of what you could have thought about looking back at it now.
Another thing you could try is working on your self esteem. People who accept others interpretations of them usually do not have a self esteem to counterbalance what is being said to them or how they are being treated. For example:
Person 1 has a low self esteem. They meet a friend for lunch one day and the friend mentions the shirt person 1 is wearing, saying they thought person 1 would look nicer in a blue top rather than red. Person 1 is insulted, as she should be, but also becomes deeply hurt, cuts off the conversation, and goes home and throws the shirt out. She never buys red tops again and refuses to see her friend for months.
Person 2 has a good self esteem. They have the same situation happen as person 1, but they react by telling their friend that they like red and the shirt is a favorite of theirs. Person 2 does feel hurt by their friends comment, but they think about the situation later and decides that everyone has said things they don't mean and lets the comment go.
Self esteem can make a big difference in how you react. It helps you engage your thoughts, gives you a basis to reflect on other people's actions, and allows you to accurately assess someone else's actions.
So in your case, when your therapist did not give you the full 30 minutes, being upset is a natural reaction. But self esteem would have told you that the situation can be handled in many different ways. Telling your therapist you are unhappy with her actions then discussing it is one way to handle it. Her actions may not have been all about you. She could have had a personal problem that held her back, a patient who had an emergency or other good reasons for her actions. Not making it up to you is not thoughtful or kind, but you may have said to her that you would like her to tack on 15 minutes to another session to make up for it or changed your payment to reflect the reduced time she spent with you. That would be a logical and thoughtful request. Self harming, while understandable in your situation, only hurts you and does not resolve your feelings or the situation.
You can learn self esteem in many ways. Here are some resources:
Overcoming Low Self-Esteem: A Self-Help Guide Using Cognitive Behavioral Techniques by Melanie Fennell
The Courage to Be Yourself: A Woman's Guide to Emotional Strength and Self-Esteem by Sue Patton Thoele
The Self-Esteem Workbook by Glenn R. Schiraldi
Once you begin working on this issue, it should help you connect with your thought process and allow you to think through your responses to others and their interactions with you.