Pretty much everyone here has talked about stuff that could be wrong with you, and it is possible, but I want to offer an alternative explanation: you aren't being challenged enough. I was in school for two years and experienced a lot of the same stuff you did (I'm 21 now, I dropped out):
1) Was smart enough to glide through classes for my whole life, college not an exception
2) Started off having a lot of fun but got over the drinking and partying phase faster than my classmates, definitely felt lack of connection with my classmates
3) Thought a lot about whether or not I was actually depressed, especially because of the lack of connection w/ classmates. Tried a lot of stuff (diet, exercise, shrink etc.) Nothing quite seemed to do the trick (though the exercise helps a ton with daily stress but will not fix depression).
Although I obviously can't recommend it to everyone, I dropped out and started working. Now I had been freelancing and doing internships since I was about 15 so I had a solid shot at an entry level position at a startup. I can't tell you how much happier it made me.
You may not be able to just drop out, but I bet if you find an internship or something that involves you in the "real world" in a way that's somewhat meaningful to you (ie: not getting coffee) you'll get a lot more satisfaction out of life.
Remember you're young, you're not exactly supposed to have it all figured out yet. Even old people don't have life figured out yet, no one ever does. You also sound pretty introverted, and self reflective. This is a really great trait, but it can be crippling if you don't know how to manage it. You'll learn to know yourself and catch yourself repeating patterns.
It doesn't sound like there's anything wrong with you to me, but again I'm just a dog on the internet. My contact info is in my profile, feel free to email me if you'd like to chat sometime.