mouse_force.y != 0 is true when you move the mouse up and down (push/ pull it from/ to you). mouse_force.x != 0 is true when you move the mouse left or right.
The code seems to move the player upwards with player_speed.x, so you need to control the latter variable with the y-coordinate of the mouse speed vector.
"Falls das Resultat nicht einfach nur dermassen gut aussieht, sollten Sie nochmal von vorn anfangen..." - Manual
