How do I know if I am in love?

I am 17, almost 18. I have known this guy for over ten years. He comes to visit every summer. We are really close and text each other all the time. I feel so comfortable with him, yet a basket of nerves at the same time, and can't imagine living without him. I think about him constantly and smile. I haven't been able to eat much of anything because I'm nauseated 24/7. I think about him before I go to sleep. He's the first thing on my mind in the morning. Yesterday he was telling my mother about this girl he is interested in, and ever since I've been depressed and quiet. My family says I've been walking around like a zombie. What is wrong with me?