So ive done this to myself. You literally get into your own head and make it worse. I've always been able to read, speak, sing, when I'm alone but when I really thought about it especially practicing for another moment I started to stutter then also. My therapist says I've gotten into my own head wether I realize it or not and projected a fear onto myself. I've still not been able to overcome this issue but I hope you have better luck. Just take it slow and remember no one is watching and listening and you can beat that