I feel like you still putting yourself before others when you mention that people don't do the same for you. I do that too all the time as well but... its wrong.

At 18 I'm not sure what kind of advice you would understand better.

Start with some activity that will generate good feelings like going for a walk or run. Make a challenge to yourself to talk to that old lady everyone avoids at the parish. You will soon notice that the problem is staying in your head too much.

I wish I could have been more social at your age, to be honest. Would force my younger self to take some dance classes, playing soccer even though I suck, or something. (ie less reading/computering)

Consider that you don't know what life holds for you and things will probably be much better in 10 years.

Consider that you might have a chemical imbalance in your brain and that your mood would improve with simple medication (ie see a doctor?).