Hi sims,

You need to very slowly push yourself out of your comfort zone. Each day or at least twice a week make a plan to talk to someone that you would usually avoid. That might be making a bit of conversation with a shop assistant or something along those lines. Perhaps make more of an effort to chat to a neighbor or someone you don't know that well.
Have you thought about joining a club, gym or some kind of other interest where you meet people?
You could also look at whether there are any opportunities to do some volunteer work, perhaps help out for free at a local youth club or something similar.
The good news is that you are still really young so if you make steps to overcome this now you won't struggle as an adult.
Remember it is the tiny steps that lead to a big change.