When I used a controller on mobile and had flight assist on it worked normal. Now when i try to use it on pc when flight assist is on if I even decelerate 1m/s it keeps decelerating by its self i can accelwrate but as soon as i let go it starts decelerating how could i have it on flight assist where as soon as i let go of decelerate it goes to normal its not a hardware issue.

This reads very much like a controller configuration error; it's happened to me plenty of times. Try going to Options -> Advanced -> Controls -> Joystick and adjusting your dead zones or recalibrating. Also make sure you don't have anything strange bound to "decelerate".