// Drops me to ground
Vector playerVector = player.getVelocity();
Vector eyeDirection = player.getLocation().getDirection().setY(0).normalize();
Double velocityInDirection = playerVector.dot(eyeDirection);
Vector directionVector = playerVector.subtract(eyeDirection.multiply(velocityInDirection));
player.setVelocity(directionVector);


// Doesn't drop me to ground
Vector playerVector = player.getVelocity();
Vector eyeDirection = player.getLocation().getDirection().setY(0).normalize();
Double velocityInDirection = playerVector.dot(eyeDirection);
Vector directionVector = eyeDirection.multiply(velocityInDirection);
player.setVelocity(directionVector);