How can a person NOT let this happen?

I shut down during my session yesterday. First time I ever did that. I was just done. I'm not one to cry in therapy, but I did yesterday, which I'm sure contributed to my "checking out." T threw a blanket over me and sat in her chair, watching me...occasionally saying something...I don't even recall what. Thankfully she I could tell she was understanding about it....I did apologize a the end, she said I had nothing to be sorry for, and gave me a big hug.

I just didn't want to talk anymore. What I wanted to do was go to sleep. Right there. Didn't matter. So, it was a complete waste of a session, and I'm sure it's frustrating to the T when that happens...and I don't want to let it happen again if I can help it. How?