Oh, tears are welling up just reading your post. I may be kind of old but I can still remember feeling just that same way when I was your age. Like I was the only person on earth who was all alone. Lots of people go through what you are going through. Eventually, you will get on the other side.
You should definitely keep a journal--just write out how you feel, regardless of how the punctuation and spelling looks when you are done. Sometimes that helps you to see what the real problem is. And try to talk about it to someone you trust--just to vent, you know? A teacher, an aunt, the social worker at school. I was lucky--I had a wonderful sister-in-law that I could say anything to.
Kristina wrote: My mom is a dear, and I love her very much, but she doesn't understand how I feel.
And, being the mom that I am, I have to say: You could try talking to your mom. You might be surprised at how much she does understand. After all, she was your age once. My daughter usually tells me things right before she goes to sleep--she will keep it bottled up and then, when I say goodnight, she lets loose with all of the nasty stuff that has happened to her that day. And, I can relate. I was an eleven year old girl once too.
But, if none of that helps, you should see a medical professional--not because you are crazy or anything, but because your life seems out of control right now. I had a similar thing happen to me after my dad died--I broke up with my boyfriend of 7 years, my brother and I were no longer speaking, and my best friend decided to no longer be my best friend. I felt so alone and helpless. I went to see a professional--a licensed social worker. And, just talking to her every week for about 2 months helped me sort everything out and regain my self-esteem.
Hope I could be of some help. And, hey, if you ever, ever need to talk to anyone, please PM me.