I think you should do what feels best however, I think it is important to be honest.

What few times I have considered canceling I told T I was in a bad space and wanted to cancel. She was supportive and said if I wanted to cancel then she understood and was fine with it. However, she asked that I consider coming in and we could keep it light and discuss not stressful things and if she asked a question I didn't want to answer to say so. I ended up going and that ended up being one of my best appointments. We discussed none of my issues but spent the session discussing other things that had a lasting impact.