Hello. Don't worry, what you are feeling is normal, many people are afraid of being in relationships. They don't like the fact that they are being with someone, that they can be pulled down by the other person's activities, or that they just won't seem as free and lively as they once were, or that problems will surface in relationships, stuff like that.
You are scared since you don't know what to do about it and he's one of your closest friends, you don't want to hurt him. And randomly he just goes up to you and tell you he freaking loves you and there's nothing you can do about it, since you don't share the same feelings as him and you are afraid to make the wrong moves and lose a friend.
Panic attacks? Maybe you are far too nervous. Have you tried talking to him after he confessed? He probably knows how you are not into relationships and yet he told you that he loves you, and expects you to at least give an answer, even if you are to reject him. And he should have at least thought of the possibility that you are going to reject him before he confessed, yet he did, so it probably shouldn't be too hard on him if you tell him you don't love him and you don't wish to be in a relationship.
Basically, don't feel too bad; some things like feelings are just out of your control and you can't help it if he likes you. Even if you are afraid of being in a relationship, does that change the fact that you are going to turn your best guy friend down? You don't share the same feelings as him so even if you are ready for relationships you are still going to turn him down--it doesn't really change things so don't feel too guilty or depressed.
First of all you've got to turn him down before thinking about your general impression of relationships, if you haven't already did. Just in case you didn't, you should tell him you are not ready for relationships. If he doesn't accept that and says he would wait, then you tell him you like him more as a friend and he's one of the best friends you've ever had, you are really sorry about hurting his feelings. Do it gently. But don't make it feel as if you are pitying him for looking like a fool by feeling this way for you. Do it in some quiet place so that he doesn't have to face his other friends and have them probe him about what happened, he probably wouldn't want that. You need to let him know that this will not affect your friendship and hope that you guys can just leave this in the past and pretend nothing happened, or if he doesn't like that, still insist that he IS(present tense) your best guy friend and this is not going to change that fact.
As for now, about you and relationship. You are probably depressed because you are turning down people and yet you don't feel happy about it. Scared to be in a relationship...would you like to define your fears? What kind of fears do you have? Is it more to the side of being attached and no "freedom", or that you are afraid to be embarrassed, you don't want to mess up in front of guys; or you don't like being too close to them because it makes you uncomfortable?
If it is that you are afraid of being attached, it is definitely not unusual and some people just doesn't like being defined as someone in a "couple". They prefer to be on their own and be for example, Jane, random name. Not Jane AND David or something. They prefer to be able to flirt and talk to other guys without fearing that their boyfriends would be jealous. They prefer not to worry about what to give their boyfriends for their birthdays, blah blah and it's kind of like...this is a pretty far-out relation, but...well, like having a pet.
You like having one, but after a while feeding it, cleaning the cage just bothers you and you would prefer not to have it and worry about it whenever you go on trips and stuff.
Another is fear of being embarrassed. Some people are afraid to screw up in front of guys they like and are afraid if they get too close to guys, they may show their "ugly" sides that guys don't like and tend to get repulsed by and basically, that they will make bad impressions on others.
Realize your fears and try not to be scared into being in a relationship. After all, it's not as if you are being married to someone, you don't like that guy, break up, find a new one...you are young, life's fresh, full of possibilities, it's fun exploring. Open up and smitten every guy you see!
Don't be depressed; go out right now and look for guys to chat to, or call up your friends and go shopping and buy some pretty clothes to wear to school the next day. And then when you are old, you tell your grandsons and granddaughters, "Why, when I was your age, I was such a diva. Everyone wanted to bring me to the dance..."
=D
Well, good luck and hope things work out.