Take this with a grain of salr because its been my experience, diaphragmatic breathing helps as much as it relaxes you but its not a magic bullet. Your stuttering might be like mine, excessive self monitoring caused possibly by trauma or just habitual. Like it started small when you were a kid and it snowballed. 1)do you stutter when you are alone? Or when you sing? 2) do you stutter more when stressed or scared? Or more with people you dont know or are hostile to you? Again its just my experience