I could call myself a loner sometimes. I don't have that much friends in college, but I mostly hang around with different people and I do have acquaintances in each of my subject.
I think focusing more on other people and why they have this and that will do more harm than you thought. I used to be like that. Although I have enough confidence to socialise, I still feel envy to those who are "uber-friendly" i.e. those who seem to have friends everywhere. But then we live in an imperfect world.
Rather than putting yourself a "benchmark" i.e. that fat guy that is having a relationship, which could only disappoint you and make you upset if you don't get any nearer to his status, why not assess yourself; focus more on things that you think will put you in a better position to start conversations and ultimately start friendship and on the factors that will contribute to an environment that will make you more comfortable in terms of socialising.
And this argument that you'll never be in this age again; that's rubbish. At this age strata, we are only beginning life mate! So what if you have a horrible teen life. In most standards my life nowadays are way way crappier than the average teenage life. It did make me somtimes depressed but that didn't make me any less human. There are more challenging things in life that lies ahead of us; and there are more opportunities in the future.