I have it return with a vengeance whenever I am stressed and tired or angry or, worst of all, whenever I try to have sensitive and difficult conversations with people in private. The reason this happens is because, depressingly, you can’t be rid of your stutter, you’re basically just holding the lid on it actively your entire life. This is pretty annoying to say the least. I also have the same habit as you when it returns. My unconscious habit is more like an ugly tic though. My face grimaces and my head jerks with the rest of my body twisting in on itself and twitching. Usually I can also keep this under control but once it begins it’s like a cascade. It’s like my entire body is trying wring a single word out of me. It’s physically painful but, above all else, it’s mentally painful. I’m not sure on how the exact ages line up but I think mine was also worse at a similar age to you. I think it’s because life is inevitably stressful during those times. For many years, I got laughed at every time I opened my mouth. At first, it was bad as a child, because I had no control over myself, I was scolded harshly and I was constantly pushed to improve. Second, it was worse when I began school at 11 because I was, for the first time, exposed to many people. Third, it was worse during my exams between 15 and 18. After that point, it never plateaued again which I will be forever thankful for and cautious of. I think it’s easiest to just accept that sometimes it will happen and to avoid bringing about the type of stress or anger that facilitates it.