mamoyo, you are arguing from personal incredulity (“this seems impossible to me, therefore it doesn’t happen”), which is a logical fallacy.
Do you believe that everyone educated in such matters has been co-opted into a conspiracy and is lying to you?
To believe in the flat earth you have to believe that every space mission, every image of the earth, every measurement of anything beyond earth, is not merely wrong but a deliberate lie that everyone in a position to know has agreed to perpetuate.
As for the atmosphere, I’m sure that at the fringes molecules do spread into the vacuum of space. But air has mass and the bulk of the atmosphere is close enough to the planet to be held by its gravity, just as we are.