Hi cutegirl and welcome!!
First of all, you are NOT going crazy! This is not your fault. And I know how you feel. You are not alone! People struggle with eating disorders everyday, that does not make you crazy!! You have found a board where people understand your fears and concerns and desires about food and weight, etc.
From the info you gave us, your friends and family are right, you are underweight. However, I know what it's like to have someone tell you that you are "underweight" and then look in the mirror and see someone who weighs 300 pounds. That's your mind playing tricks on you. It's a mind-game, and beating the demons in your head is the hardest part about overcoming an ED.
As for whether or not I think you're anorexic, you know in your heart that the only person that can tell you this and make you believe it is...YOU.

I'm not a medical professional, but I can tell you that 900 calories a day is NOT enough food for a growing person. You're only 13, your body needs more food, not less. I know me saying these things doesn't make a difference. I'm right where you are, it's easier to GIVE advice than to take it. But if you think you're ready to get help then I say TAKE THE CHANCE. It's always better to get into recovery as soon as possible! There are so many horrible things that go wrong with an ED (as I'm sure you know, sounds like you've been doing your research! That's awesome! KNOWLEDGE IS POWER)

hehe, ok, now I sound like a "stay in school" commercial.
anyway, let us know how you're doing. You've found a wonderful board with many supportive people who can understand what you're going through when it seems like you're totally alone!!!
NEVER GIVE UP
~sarah~