Fake it 'til you make it.
Obviously it isn't all you should do, but being downtrodden and depressed will make it harder to make friends, make it harder to bother doing anything productive, and it is possible you could burn-out from lack of emotional fuel. So if you really want to get out of your bad head-space, you need to pretend that you are, even if you are the only one that doesn't believe it. If you put pretending to be happy first, you'll be more likely to make good first impressions, you'll be productive (maybe get a hands-on hobby), and eventually, one day, you'll realise that you are actually enjoying all these little things (ad that day may be a week from now, a month or a year .... maybe more, but the point is to try, because you don't get out of downward spirals by turning off your engines and letting gravity take over.
OT: As for an example. When I started uni, it was such a radical shift from high school, and most of my friends were at other unis, or TAFEs, or had jobs, and most of my classes were so big that it was hard to make new friends. I slumped, and I slumped hard. This semester, I put myself out there, pretended to know what I was doing (socially and academically), and you know what? It worked! I've made 10 new friends, I'm going to all my classes, I'm generally happy, and while I still may be a teenager dealing with the emotional side of his hormones, I'm doing loads better than I was last semester, and it is all because even though I was (and am) unsure, worried, over-whelmed, a little scared, and more than a little insecure at times, I pulled my finger out, said "Screw feeling this way" and made myself appear like I was the man I wanted myself to be, even if I'm not there yet.