ive had a similar question ( i want a max speed and able to axxelerate in antoher direction even if the max speed is reached so i will answer this questiion in some more detail:
for your question, what chris means was somethign like this:
note that i set the angle to -180degrees to deaccelerate in the opposite direction.
in addition to this i faced the problem with the max speed which is usefull for any space simulation. i just don makes sence to dont cap the maximum movement speed. so here is my solution to this which works pretty well
I hope this helps