I'm sorry things got moved around. I would find that very upsetting too. I usually need a week or two warning for me to be OK with a change in time or date. consistancy helps most of us feel safe. Some people need consistancy in different places or ways than others and some people need more while others less... but we all need some. It might be important to tell your T that the standing appointment is an important part of your feeling safe to do the work you need to do.
I know it sounds funny but one of the people I work with is moving from her summer clothes to her winter clothes and even that throws me off! It is strange what comforts us but it is what it is.