The feeling itself is important, and is worth talking to your T about. Even though you understand that she is doing her best to make sure that she can still see you on Mondays, and you would adjust to seeing her on another day, you still feel pretty stressed thinking that you might lose her. That sounds like a powerful attachment, which is part of how therapy works. It sounds like you are important to her and I'm sure that she will make sure that she is able to still work with her. The important thing here is how you feel.