Depression nervousness. To look at her?

I am so much depressed. I think its a situation that i can't move on or stay back. Its a problem that i have been facing from last 3 years. Its about relationship. I love one girl who is my best friend. & i wanted to happen things between us,but it should not happen because of some limitations we have. I am insanely in love with her from last 3 years.she does share her feelings with me like she is having crush on guy etc. Wich kills me from inside. I have her fb password. I read her chats with guys. & get angry . now she is ignoring me wich is.....how can i change feelings for her like a good friend. i lost my 3 years just by thinking over over about her. Now i hate to see couples. Whenever i see any movies i saw her in actress & any random guy in actor who does Romans. World became hell for me. How can i change the way to look at her?