where you just feel like not existing?

You don't. The entire world is just my imagination, and you are a part of it.

in school

Really? Oh, ok! That makes it a little easier to deal with.

I'm probably pointing out the obvious, but the above is impossible. If you don't exist, then there's nothing there that could have any feeling. Hence, the concept of a "feeling of unexistence" defies itself.

What do you mean specifically, though?

ehh, I'm just depressed basically :P

non existence does not implicate nothingness.

For example, unicorns doesn't exist, but as a concept, they are in people's minds.

Like all you people exist as a concept in my mind. That concept is so powerful that you truly believe you are real.

You stole my idea! ... oh wait.

My head hurts. :(

Actually we are a historical simulation of earth in the beginning of 21st century on some kid's laptop in distant future. We are just npcs. Preprogrammed to believe that we are actually alive or have a free will. In reality tough, we are just tools used to entertain and educate higher beings.