No kid deserves this kind of treatment. I have no way of knowing if your mother is insane. Based on what you wrote, I do think she is abusive. Regardless of the reason, she had no right to hurt you physically in the past and has no right to humiliate you or shout at you now.
It is no surprise that you are feeling depressed. You have been repeatedly traumatized by the person who is most supposed to love and care for you.
You can’t help your mother. But you can help yourself. At 18, you are legally of age and can seek out treatment for yourself. Find a therapist who specializes in trauma. Ask your doctor for some names or talk to an adult you trust at school to find out where there are programs for teens who have been hurt.
While you are looking for a therapist and beginning treatment, limit your time with your mother as much as you can. Get an after school job or join an activity that will keep you out of the house. Start thinking about ways you can leave home and be on your own. You are being regularly retraumatized as long as you live with your mom. This is only going to make you feel worse.
You are also entitled to stand up for yourself. If you mother starts to shout at you or embarrass you, calmly inform her that you won’t listen to it and leave. Go to another room. Leave the house if necessary. Do not yell or shout or argue or behave like she does. That only keeps the negativity going. Just quietly leave. She may eventually understand that you won’t stay around for bad treatment.
Another way to help yourself is to join one of the forums here at PsychCentral. People with similar issues support and encourage each other.
I wish you well.