Don't just rely on therapy, try to help yourself too. Figure out what makes the improvement possible and try to bring it to other contexts. e.g. I usually have a few close friends I'm much much better at speaking to, so sometimes for a few seconds I pretend I'm speaking to them instead. Or during a presentation, I'll ask a friend to make extra eye contact because it helps me focus, whereas while stuttering I usually avert my gaze (as do most bystanders).