I hate this

I agree about not telling a peer.....

It's good to tell someone, but think carefully about who that someone is.

Well, when I was about your age I told a classmate how I felt. It went much better than I ever thought, the person I told was very understandable, and said lots of nice very nice things to me. But it wasn't enough. That person couldn't do anything more than comfort words.
Later my parents started to realize that I was not very well (allways in bed, not wanting to do anything...) and my journey started...the journey of going through doctors and psychologists. It was a very though thing to do at the time, specialy because the first time they schedule me an apointment without telling me a thing, so when I realized I was in a mental health professional (not that I dind't already want it).

At the time, and still now, I know there can be many ways to get some help, even without telling or parents. But for me the best way to get it, was to at least give my family some clues about how I was feeling (little by little, and not telling them the all picture). If you think they would support you, they would understand you and they want the best for you (what should to be a condition of being a parent), in my humble opinion I think you should at least tell them something.
I didn't knew what to say too, and specially I couldn't make myself tell them I wanted help, so what I did: start to complaining about little things (my memory isn't working, I can't go to the exam, I will fail, don't be afraid to start crying in front of them...in my case of course) and it worked, very hard and took me a long time, but it worked. Surely not the best way to get things donne easily and quickly.

I was wondering why shouldn't I tell my friend?

Well.....
Mulan has already given you her experience.
It will feel good to share, but ultimately, she will not be able to do anything other than listen. In fact, at that age it's possible that she will be scared off, scared for you and tell her parents, you may have an argument and she could tell others (and warp the truth)

I just see no benefit besides you letting it out....and I see risks.

Just my opinion, of course.

Well...We are like best friends, we have been since 3rd grade, & I don't think she would ever tell anyone anything.But, I could be wrong.

I def. wouldn't tell any of my other friends, though. I wouldn't trust them.

I disagree. If it is a friend you totally trust then at least you are not alone in knowing this. It breaks the ice. You share part of your burden. Maybe she cannot help you but listening and understanding and not judging are very powerful. Of course you have to be confident she will understand and not judge and not tell anyone. So I guess there are risks. We have to take some risks though in getting help. Maybe it will be easier to tell a counseler or your parents after you tell your friend.

At sixteen, things change...
Once told, you can never UN-tell.