I feel like I'm the only person alive in the world.
Why do you think that no one else is alive? Do you think you have an undiagnosed mental disorder?
I don't think so, it's just something I feel to be true.
Putting so much importance on your own existence may not be worth your time and you should not feel so much more important than others. If you think too highly of yourself others will see you as pompous.
Sheesh, I just want to think about my own existence for a minute.
Well, it's okay that you think about your own existence, but doing so, makes others feel hurt as they have the same feelings and goals that you have and you are diminishing their importance if you think you are the only person alive. I do hope you change this behavior and think about how you can improve as a member of your society.