Firstly I don't see the failure. You took the courage to talk to a girl you like which is hard to anyone, let alone us who stutter. That's a success in my book already. I don't know if she was trying to flirt or doesn't know how to but it was a pretty immature joke and I'm sorry you had to deal with it. These kinds of experiences aren't fun, but they do make us more resistant and tougher in the long run. Keep talking to girls and you'll find one who's more mature.