An example of new car syndrome:

Dear Carolyn: I sit here at the beginning of another year of law school, but find myself feeling incredibly alone and unfulfilled. Most of my friends from college have gone their separate ways, gotten engaged or married, gotten used to me being elsewhere, and moved on.
I'm in my mid-20s and I feel like I've accomplished nothing. A terrible bout of depression kept me from really engaging in anything substantial in college, and these days, the things that used to interest me no longer do. Maybe people are just better at faking interest at what they do than I am, but if you told me when I was 15 this was what my life was going to look like, I'd have been shocked. I just don't know what to do. I'm tired of a life unlived.
-- Va.

Dear Va.: I hope you're still in treatment for the depression, at minimum having someone you talk to who can monitor your condition.
As for the lives your friends are living, please don't put too much stock in appearances. People are up, people are down, many are both at the same time -- but few are letting it all play out in full view.
That's why looking over your shoulder is of very limited use in assessing your place in life. Not only are you not seeing the whole picture, but it's also someone else's picture. The only thing that matters is the way your life fits when you're the one wearing it.
If your life doesn't resemble what you had in mind, you're in plentiful company. Depression can certainly add to any disappointment you feel, too: It affects both your ability to enjoy the moment and your ability to see where that moment is leading. That's why treatment is so important. It can help you sort out whether it's your life that needs fixing, or just your depression-tinged perception of it.
I also would suggest making basic, physical adjustments to boost your outlook, and specifically help fend off depression. If you're healthy enough for it, get regular exercise. Get enough sleep. Eat well. To the best of your ability, will yourself to do these things.
Then, slowly, one move at a time, start looking around for fulfilling new things you can do with your time. If nothing appeals to you, try do-gooding just so you get the buzz of doing good. And so on.
And finally, give yourself a credible narrative for where you are in life right now. You haven't failed to live your life, and it's not as if you haven't accomplished anything; you're simply immersed, temporarily, in the pursuit of a long-term goal. It's not failure to bloom, it's a bloom of delayed gratification.
If your concern is that the gratification will never come, start taking small, practical steps toward planning your life after graduation. Ask yourself what you would consider a fulfilling life purpose -- advancing a cause, say, or protecting the vulnerable, or making great piles of money -- and then narrow your focus on the career path that would take you there. Having a steeple to chase, even if you end up changing direction later, can do so much when you feel a bit lost.