I've always been naturally social, but only recently have I been genuinely more positive, so this may or may not help you:
I basically changed how I see myself, the world, and other people.
I have depression, but I've been working on it by improving myself regularly. Every aspect of myself I dislike, I find a way to work on. That way, even if I can never really reach an end goal of what I want to be like, I can see myself progressing. I might not be a 'good' person, but I try--and that's how I see myself: someone who does the best with the shitty hand he was dealt and makes it work for him.
As for the world, I've started to see it more from the view of the Stoics. Nature does whatever the fuck it damn well pleases (Nature with a capital N, meaning the force that controls the universe, not 'nature' as in the trees and shit) and there's nothing I can do about it. So, I should only worry about things I can control, because worrying about things I *can't* control will only cause undue stress.
And as for other people: Instead of getting angry with stupid people, for example, I recognize that they can't be anything but what they are--I don't have to respect that, but I understand it--so if they don't listen to reason, that's no skin off my back.
All of this has made me a less angry person in general, and my negative emotions have been blunted. So, I can be more optimistic and content and "radiant/positive" without faking it.
TL;DR change your attitude about yourself, the world, and others, and you'll probably be better off for it