Mental state can play a big part, but the short version is that stuttering is caused by a disturbance in the execution of planned movements. That is to say, your brain knows what it wants to say, formulates a plan to say it, and then there’s an issue actually performing that plan. We don’t know why this is, yet. I don’t think I fully understand what you’re meaning, but it sounds like you’re saying you’re fluent when you’re barely able to breathe. This isn’t super surprising; whispering is a common condition for fluency. I don’t know if there’s a consensus for why yet, but I’d imagine because it’s less of a cognitive load not having to coordinate with the larynx to produce voiced sounds.