One thing that I find helpful is to sprinkle red clay at the threshold of your door. I know it was used in the skeleton key, but I've been using it since long before that movie came out. And like she said, just talk to your mom. Sit down with her and just talk about how you feel you need your own privacy.
Now in regards to witchcraft, if your mom is very hardcore bible thumping christian, then getting her to understand could very well be out of the question, but if you ever feel he urge to bring up the subject, allow the goddess to guide your words. I know i was in the same situation, but one day me and my mom were sitting outside and something quite literally forced me to bring the subject up. I don't really remember much of the talk cuz it felt kinda like a trance in a way. If you can do this I would also try and get a few books, specifically (although I can't stand her in the least) the book Teen Witch by Silver Ravenwolf. That's what I used. Hope this helps
May the love and light of the Lord and Lady be with you always,