hi zoo,

what if you call and say, "hi, i'm actually calling to cancel. this is really hard and i'm scared and i don't want to come tomorrow. i just thought you should know. i'll be there because i've never not been there for session, but if i could have it my way, this would be my call to cancel."

that way a) you got to say everything that you wanted to say but b) you're still going and c) she knows what to expect when you get there. i think that when she opens the door to let you in, she'll have a really good impression of where you're coming from, and will know to be extra gentle with how you're feeling. just an idea. otherwise..

many hugs to you!