Hello, I know dealing with crushes can be an extremely rough thing to deal with - especially if you've liked the person for a long time without the other person reciprocating those feelings. You really have to understand that it's all apart of growing up. Teenagers will experience crushes, dating, break-ups, heart-breaks, etc. You just have to learn to cope with these sorts of situations.
If he just wants to be friends, then you'll just have to respect his wishes. You can't force him into anything that he feels uncomfortable with. I am not sure exactly as to why he kissed you. I don't think it's a pity kiss, because obviously this guy does like you, or else he wouldn't have kissed you.
I understand you feel really heart-broken right now, but you really have to move on. As hard as it may be to do, it's something that has to be done. Have you talked to him about the reasons why he just wants to be friends? In many cases when it comes to best friends and relationships, one person usually values the friendship so much that they don't even want to run the risk of ruining the friendship. If this is the case, then you should be extremely honoured that he values your friendship as much as he does.
I understand that this must be really difficult for you, honey, but you can't get depressed over it. I know you really like him but there are many other guys out there for you. If this doesn't work out, don't get down on yourself. Keep your chin up because there is always someone else out there for you! Don't give up. Maybe you two were just meant to be really great friends - and there's nothing wrong with that. Sometimes, that can be better.
Just think about it: You guys have been best friends for 3 years now. If you guys went out and then broke up, what would happen to the friendship? Usually after people date, the couple never talks to each other again. Is that what you want to happen? I know it doesn't happen like that all of the time, but would you actually want to run the slightest risk of that happening? I'm sure you wouldn't, considering how much you value your friendship with him. And that could also be the reason why he just wants to stay friends right now.
It's completely natural to feel somewhat depressed and want to cry when you see him. But, you have to realize that both people have to be comfortable in the relationship in order for it to actually be a relationship. If he doesn't want the relationship, you can't force him into one. That's not right.
Do you have any friends that you can talk to about this? Sometimes talking to close friends about your problems - especially problems like this - can really help a lot.
Here are two great links that I strongly suggest that you read up on. I'm sure you'll find them extremely helpful.
How to get over your crush
How to Deal with Crushes
I hope I've helped. If you need to discuss this further, feel free to message me any time. If you ever need anyone to talk to, I'm always here to listen.
I wish you the best of luck. Keep your chin up!
~ Sabrina