Find a different speech therapist. Any therapist that sees you for that long and isn't achieving results should say "What I'm doing isn't helping, let's find someone who is a better fit for you." There's a genetic component to stuttering, but that doesn't mean you can't improve fluency. I was a severe stutterer. I achieved fluency to such a degree that coworkers don't know I stutter. And I talk a lot at work. 18 is still young. Your brain is still plastic. I went back to speech therapy at around 25 when I got fluent. Inadequate speech therapists go a long way towards convincing PWS that they just have to accept their stuttering. Pisses me off.