I just made sure that the Doc and DMV did it, not me.  Even
though I ghost wrote the letter for the doc.

She will keep forgetting that her license was cancelled for
medical reasons, and you will have to keep remininding her.  You
need to get used to being asked the same question over and over.
As her memory becomes more impaired, she will be less and less
able to remember what just happened, though she may remember
things from 50 years ago like it was yesterday (which will, of
course, get her more confused).

Be thankful the Doc did this.  The usual question on this topic
is how do we take away Dad's keys.  Best to sell the car, get it
out of sight so it doesn't remind her of driving.  Don't forget
to go to your DMV and get her a state ID to replace the driver's
license.

Go buy a copy of "The 36 Hour Day" which can tell you some of
things you can expect, and attend that support group.  You'll
find that you're not alone in having to cope with parental
disabilities.

Good luck, and feel welcome to come back and ask questions or
vent.

Use the force.
Tell her the car needs a little work and it'll be ready next week.    
This should make her happy, and the AD will ensure that by next week
you can safely use the same story again.
Yes, it's a deception, but if it makes her happy and stops her fretting,
there's not only no harm in it but it's actually the best thing _for her_.