I used to think about death all the time too. And that was when I was like 8 years old or something. I'd wake up in the middle of the night, drenched in cold sweat thinking "I'm gonna die" and thoughts like that. But it passed eventually. Just don't make a big deal out of it. Personally, everytime I started thinking about these things again, I would think about a happy thought. And the thought can be anything: a clip from a merry cartoon, a memory of your friends... It worked for me.