It's damn hard not to think about it. With practice, you can reduce the amount of time you're dwelling on unproductive / harmful thoughts. Start with recognizing when you're doing it. Then ask yourself how much time you're going to allow yourself to think on it. It's perfectly normal to reflect and think on such things. But set yourself a limit. How long are you going to spend on this? Only you can determine what's the correct amount of time. If you decide on 30 minutes (just an arbitrary number for discussion right now), then give yourself that 30 minutes to work through it. But at the end of the 30 minutes, you have to shut it down. Move forward. Do this instead of allowing it to be always present. Give it the time you feel it's due. And dedicate that time to it. Exhaust the thought process. Then move on. With practice and discipline, you will find yourself able to spend less time dwelling on your stutter, freeing up more time for the rest of your life.