Wow - well, good for you for speaking up. Teens often think they need to protect their parents (I was one not too long ago). And you just said it - they think you are this happy go lucky person and it is all a lie. That is why so many teens end up committing suicide. I am VERY GLAD you are going to talk to the school psych about this. Yeah, things will be different - but they clearly need to be if you are feeling you have to live a fake life. And yes, your parents may become over protective of you, but once you are able to establish who you are, you will also be able to have a voice that can be heard and clearly tell them what you need, how much space you need - etc. Your parents may also brush it off - just something to think about. And is there any chance that your twin is really actually afraid of this bringing *her* stuff out in to the light? It seems odd that you would be going thru this much stuff and for her to worry more about mom than you. But that's just my take on it.
As one who did always hide everything from mom and always lived a fake life, I am here to tell you that I think you are very VERY brave and taking a huge IMPORTANT step. If more teens spoke out and reached out to professionals like you are doing, there would be far less teen suicide. It is a very real issue that cannot be overstressed.
Good for you!!! Have courage!!!