I’m 37 and have had a stutter my whole life, I found mantras helped me big time. I speak clearly, my voice flows like a river, everyone around can hear me clearly, I can speak my thoughts, I am strong, I’m confident. Saying these phases over n over again for a couple of minutes really helped me and is helping me to this day. Sounds silly until it works. Also for negative thoughts I simple say “CANCEL” in my head or out loud and it helps to kick those negative thoughts too.