I'm 21 so not quite your age, but close enough I guess! First of all I really feel you there with having people say "hello, hello?" on the phone, etc. It's always a pain. Regarding the head jerking thing, it's relatively common for stutterers to experience secondary behaviors. I have twitches as well when I stutter - not always, but often enough to make it a concern. I used to scratch my arm and it was brutal. Also had a type of head jerking that was more of a repetitive nod. Currently my secondary behavior is where I open my jaw all the way (to force through a block) and the pressure gets really painful. It's very hard to stop because it's involuntary, like your twitch seems to be. As much as it is easier said than done (that's for sure), my best advice is to just stop what you're saying and restart. Like I said because it's involuntary it's hard to just stop, but restarting is the only thing that has seemed to help me over the years. But to be frank, I can't say I have a cure. Twiches are extremely hard to treat, but you can definitely look into it some more, and make sure to emphasize it even more with your speech therapist. There's always a chance it could get worse so don't let them glaze over it. I wish you the best of luck! Keep us updated on anything you find, or your progress.