Anxiety Support Group
Anxiety is a physical condition marked by intense and persistent feelings of distress, fear, angst or dread. General anxiety caused by routine day-to-day stresses usually passes quickly and is experienced by almost everyone at one time or another. However, such feelings that linger over time and are very difficult to cope with, and which lack a clear cause, may indicate...

Is it possible for you to set up your own appointment and make your mother the first topic so that you can be a little more prepared when your mother figures out?
Just wondering. . . Continued support while you find what is right for you. An adult really does have a right to privacy.
She's not the issue, the family with the mentally ill son is,
I just think my mother is afraid I'll volunteer info about her,
Your mom's attempt to control what you say tells me a lot about her self. That is not your problem. You have the right to take care of you, and to learn how to be constructive, positive, firm, and separate from your mom's view.
You are responsible for what you say, do, react, and respond. You are NOT responsible for what others say, do, react, and respond. This includes your mom, and if it gets worse before it changes for the better, that is her problem, not yours.
It takes time to work through the stuff to sort out how your thought process works.
You've taken the important first step in writing it out here. Keep working it out!
I don't believe you need to discuss it with her at all or run it by her first, but if it's nagging at you, maybe a way to just get over the stress of her finding out is just getting in front of it by telling her what you're doing (once you resolve to do it and have scheduled the appointment).. That way it takes all the stress and fretting about trying to hide it out of the equation. Maybe it's just saying without skipping a beat, I'm going to therapy to work out some stuff. It has nothing to do with you. And then discuss it no further. If she pitches a fit or gets weird about it, that's her deal not yours. She'll eventually deal with it one way or another. And even if you do talk about her during therapy, and you will, because we all do eventually, even if one had great parents, you aren't obligated to share that with anyone outside the therapeutic process. It's not lying; it's not hiding. It's private. Period.
You love your mom but she's an adult. She just has to deal with it.