Not sure what the deal is with people attacking the OP, but it's obvious the guy just needs a little reassurance. I can empathize with you since I also feel the same way sometimes. Life does seem mediocre sometimes; being on auto-pilot like you said. The only difference between me and you is that I'm 24 and you're 19. You have so many years to shape yourself and your life before you should even begin to feel like this.
It's also like what another poster said; we tend to give certain things more thought than they deserve. Sometimes things just seem bleak and hopeless but that's just a state of mind. You need to figure out why you are feeling like that and deal with it or else this will continue to be reoccurring. Below is a link of a really good article on dealing with emotions. It's a three part article that I think might help you.http://www.trans4mind.com/heart/emotions.html
Combine that with a good diet, going to the gym on a regular basis and you'll find that you feel better for the sole reason that you are doing everything in your power to make your life better. There truly is something to be said for giving it your all.
P.S. Getting laid might send you into a state of delusional, ephemeral ecstasy, especially since you are susceptible to it, but it won't solve the underlying problems that are causing your cycles of happiness and depression.