i was in speech therapy since I was 4 and the only thing which ever helped was the absolute basics. **Slow down.** It's cliche but it's the only thing which actually helped me to improve to the point in which my stammer is undetectable. I also sorta developed my own technique of extending stammers instead of fighting them. For example, if I know I'm going to get stuck on 'history' instead of fighting the 'h' sound I will extend it as long as it takes without going over the edge and stammering until my brain can relay it and I say it properly. Eventually this worked and I managed to go from a fairly bad stammer to one which was barely noticeable. msg me if you have questions.