If you are having a hard time slowing down your board, was it because you were going too fast and experienced Haptic Buzz/Pushback?
It can be common for riders to counter pushback by unconsciously shifting their weight forward to bring the board back to level. By leaning forward, you are requesting that the board accelerates more, and stop the board from decelerating. That is why it seems like you can't slow down.
If a rider is fighting pushback or is experiencing extreme pushback from the board, the nose of the board will tilt up and lower the back to encourage the rider to lean back and slow down, however, this does not work rider keeps their weight towards the front of the board.
The best way to handle this is to relax your hips and allow the board to push your weight back and slowly brake.