why don't I stutter when I read or talk to myself?

why don't I stutter when I read or talk to myself? Today I decided to read out loud. It helped me a lot to understand what I read and, above all, I realized that I don't stutter. In my case, I only stutter when explaining something, when I feel nervous and sometimes the words don't come out, but if I say them in a low voice, almost like whispering, I can pronounce them, but not in a normal voice. I hope that this practice helps me and I wanted to know what other tips you would give me to improve. I know that mine is a psychological problem.