pdx and pdy are the speed at which the position of the ship changes, so that's why it accumulates. When you press forward, the speed (pdx and pdy) become larger numbers, so you end up going faster
...and faster and faster and faster, and there's no way to model inertia when you change direction with this "direct" approach. If you start thinking about those Sin and Cos calculations in terms of generating a thrust effect which can then be applied to th ship's x and y velocities, things will fall into place. By the time you're through I'd be very surprised if you hadn't renamed pds# and changed it from a constant to a variable.