Or, is it simply a character trait that, for the most part, can't be changed?

I've tried different ways to inspire myself to think more positively, but it's never a permanent thing. Something may inspire me for a week or two, but it never lasts. But of course, this is just my experience. Has anyone managed to changed the frequency of their negative or positive thoughts?

Edit: Come to think of it, I'm more concerned with other people's experiences than whether it's "possible" or not.