Whats the best place to tell my mom something important?

You need to tell her that you want just a mother and daughter time. Where you can have her all by yourself. Tell her yo have something very important to tell her and this is for her ears only. Then sit her down and tell her everything. This condition that you have is not healthy. Being the very smart girl that you are with your mom's help you can face this together. I will keep you in my prayers.