You do have a choice.

My T did that to me once...sort of. She asked if she could move my appointment back by 2 hrs so she could fit someone else in. My thoughts were: "Well, I guess this other appt is more important than I am. This person gets extra time and I'm the one that has to compromise." It turned out that she knew I would be flexible with her, that if I really wasn't okay with I would say so, and that the other appt was just an assessment/testing which takes more time.

If you want the appt, say so. You can tell at the same time that you feel guilty for taking a time slot that she could have gotten paid for. She offered it to you for a reason.