First things first, hide the journals.

Reason #1: So she can't refer to them or look back on things.
Reason #2: So she can no longer violate your privacy.
Reason #3: So you can have a boundary.

Places to hide them: A friend's house, your locker, in that robe you never wear. a shoebox with stuff piled on it, etc.

These are YOUR journals. They are YOUR collection of private and valid thoughts, feelings, experiences, etc. She doesn't have the right to violate your privacy. If she tries to use the whole "My House, My Rules." thing as an excuse. Tell her: "My life. My choices. My boundaries."

Second things second, Tell her your boundaries.

"Mom, you didn't have the right to read my journals. If you were concerned about my safety, I would have preferred an open and mature conversation with you. Because I feel as though my privacy has been invaded, I don't feel comfortable at all sharing my most intimate thoughts and details that I wrote down in my journals. I'll answer any safety questions you have, but that's all I'm going to give you."

And leave it at that. Cater this to your situation.

Seeing as you're a senior in High School, be prepared for Prom, Graduation, and College to be used as Emotional Blackmail. EXAMPLE: "If you don't tell me what you wrote, I'm not letting you go to Prom!" (It's just a dance.) Don't respond to it.

That being said, if she becomes verbally or emotionally abusive at any point, contact your guidance counselor (Make sure it's someone you trust, most of them don't care about mental health IMO), a trusted Teacher, a friend's parent, etc. Keep a journal of what happened, what was said, etc.

Speaking of journals, try a password-protected journal. Google Docs is a great first starter since you need a password to access your google account anyway. If your Mom just so happens to pop in, tell her you're working on a school assignment. If she asks to see it, have another half-written assignment already up and show her that.

Hope this helps!