Atleast she was blunt with you. It's better than getting ghosted and not having any closure. And I don't think she is "immature"... she is just someone who prefers not to deal with a bf who stutters. Just as you will likely not also date a blind, cripple or mute person. Not everyone will accept you the way you are. That's just the way it is. Hope you find someone who will be more accommodating. All the best.