Hello there, and sorry that you've been going through all this...

Loads of excellent advice from others, but I would like to add a little if ok? Leaving someone like this takes courage, but I agree with the others, it sounds like it will be your only change of a calm and peaceful life. You deserve so much more. Am glad too that you are thinking about how to go about it, it's brave and a really good important step.

While you are thinking, it may give you the opportunity to start planning too - even if you don't go immediately. Do things like set up another bank account secretly, and try put any spare dollars your mum leaves with you in there. If and when you do go, you could also ask work (if you work) to pay your wages in there.
The other thing that's worth thinking because you mention your belongings, is to plan to leave when your mum is out the house.. that way you can leave without her stopping you (or knowing in advance) and be able to grab a couple of bags of your things. I realise you probably can't take it all with you, but you will one day, be able to replace things when you make a new life for yourself, so please take comfort in that. And they are just things, your mind, your welfare are much more important.

Please don't worry about what the police would think of you if you did say something. They would've come across someone in your position before and they are not there to judge you anyway. If you do need them, please don't be afraid.

Please look after yourself, and am wishing you all the best. Things will get better.

Hugs.