When I was a teen I went to a two week long course held by Ross Barrett. He's currently the Director at the Eastern Virginia Stuttering Center. I was 12 maybe 13 at the time. The class was mainly adults, and being barely a teenager I only gave it about half of my all. It did help though, it reduced my anxiety and lowered my stuttering for a good few years. It probably would have helped a lot more if I had gave it my all. He offers refresher courses too if you ever feel the need to go back. Here's a link to his website. https://www.fluentspeech.com/