I do not stutter so i cannot understand your situation, but I think the teacher had a good intention. so just like the other commentors, I would say that you are not stupid. You are not incapable. You speak differently and whoever has a problem with that is a major asshole. Also, the teacher gave you a choice. You could choose to continue or to step away. I dont know if what you wrote here is all that has happened or all that she said, but supposing that it is, then she didnt tell you to step aside, she offered you the choice to do so as she saw it was hard for you.