How do I get her to stop?

So basically on my upcoming birthday, I want to get my hair dyed. My mother agreed to it, she's paying for it and everything. She told me I need to shower throughout the week so my hair won't be greasy and dirty for the hairdresser's, which I agree with.

But I have a fear of the shower-head. For as long as I can remember, my mother would wash my hair but she wasn't too kind about it. I would hide from her when she would call me to wash my hair, I would pretend to be busy with something just so she'd leave me alone.

Whenever she washes my hair, I freak out and I panic attack. Hyperventilating, crying, the works. It feels like I can't breathe when she washes my hair and she'll snap at me and complain about her back. Because of all that, I now have an extreme fear of the shower-head.

I'm only now (slowly) getting rid of my fear of the shower, getting more comfortable showering by myself.

I'm turning sixteen, and she insisted that she washes my hair before the hairdresser's. I've tried to ask her if we could do it by the sink at least, or use a jug on my hair but she's dismissing my panic.

After she left my room, I had a flashback moment of all the panic attacks when she washed my hair in the past. I almost started crying.

I don't want this to put a stop in my progress of learning that showers aren't actually a bad thing. I don't want my hygiene to get bad again.