How did Columbus know the earth was round? Easy, there was plentiful evidence, from the curve of the horizon to the way ships came over it when approaching to the calculations of scholars going back to the ancient Greeks.
Why did Columbus believe the world was much smaller than it really is, and the distance to Asia therefore manageable for men in the small merchant caravels he proposed to use to get there? Because it fit his preconceived notion, and if it were true it would have been convenient for him.
Your perception of reality is distorted by misery. Don't trust it.
