Well, obviously Dallan, one thing you could do is go see a psychologist or psychiatrist. I'm not sure if anything actually beats professional help - after all, if you had a broken leg, say, or chronic headaches, seeing a doctor is probably more productive than talking about it on an internet forum. Not that forums aren't helpful - very good for venting, getting general advice and such. Of course, that may be a bit too expensive right now - and then internet forums like this ones are a good second-best.
Anyway - at 18, it's a bit too early to start thinking that life ain't worth living, that you'll never get a girlfriend, etc. Sure, if you keep telling yourself these things, they'll probably end up coming true - so maybe you could try telling yourself something else? For example, you could make up some affirmation-type things that you'd repeat to yourself every day. Like just write them down on a little piece of paper, maybe only ten or so of them, and look at it once a while. Or make a bunch of them, the size of a playing card, one affirmation on each, and when you're feeling down, you pull one out and read it (aloud or quietly, I guess it doesn't matter). A friend of mine did a similar thing to stop smoking (among other things) and he claims this helped a lot with that.
Well, it's an option - and there's an awful lot more of those lying around, waiting for you to pick one (like the many that have already been suggested above). The main thing is to get rid of all this negativity (preferably without drugs, unless a doctor has prescribed them) - it really does have a tendency to turn into some kind of nasty self-fulfilling prophecy. It might also help to get rid of this "I'm a kind of person who needs someone to love and be loved by" idea. I mean, of course, we all (or most of us, at least) need other people around us to love and touch and all that. They make life better. An old analogy may be appropriate: These other people may bring spices (like pepper or cinnamon or whatever), but we gotta supply the meat and potatoes ourselves.