Don't ever voluntarily leave a job without having another job to go to. If you do that you forfeit your rights to claim unemployment insurance.

Go to the doctor and get a letter from indicating the degree of your disability and speech impediment.

Write a letter to your manager and include a photocopy of the letter from your doctor.  Explain to her that you enjoy your job, but you are concerned that they are going to push you into an area of work that you are neither comfortable doing, but also are incapable of doing according to your doctor.

There are laws against firing people for a physical/mental disability.  They should be able to find you work within the company that is within your ability.

And if not, they will lay you off, which would entitle you to claim unemployment benefits.

But don't quit!!!!!!!  Don't quit!!!!!!

"It is good to see ourselves as others see us. Try as we may, we are never
able to know ourselves fully as we are, especially the evil side of us.
This we can do only if we are not angry with our critics but will take in good
heart whatever they might have to say." - Ghandi