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Maybe all you have to do is put a ELSE conditional to set Zero into X axis when no button is pressed. So, when the key left up, the actor stops.

//Handle rightward movement
 else if(Sup.Input.isKeyDown("D")){
 new_x_speed = this.speed;
 } else {
   // Set here, inside the else.
   new_x_speed = 0;
 }
(+1)

I discovered this, it was a case of the stupidity that you miss in the moment. The speed the object already had was read as the absolute value of. Thanks though.