Forget the pot for now
What happens if you write 45 to the ESC ?
What happens if you write 135 to the ESC ?
Does the motor then run in opposite directions for each of the values ?
Also check that write(90) or some number very close to 90 will stop the motor.
Other things you can try are taking the 1000, 2000 out of the attach() and turning the potentiometer SLOWLY. The Servo library and most servo testers work over a greater range than 1000 to 2000 and many ESCs will refuse to reverse direction if you try to go instantly from full forward to full reverse without a pause at stop.