Friedrich Nietzsche writes this about his experience of inspiration in
Has anyone at the end of the nineteenth century a clear idea of what poets of strong ages have called inspiration? If not, I will describe it. - If one had the slightest residue of superstition left in one's system, one could hardly reject altogether the idea that one is merely incarnation, merely mouthpiece, merely a medium of overpowering forces. The concept of revelation - in the sense that suddenly, with indescribable certainty and subtlety, something becomes visible, audible, something that shakes one to the last depths and throws one down - that merely describes the facts. One hears, one does not seek; one accepts, one does not ask who gives; like lightning, a thought flashes up, with necessity, without hesitation regarding its form - I never had a choice.
A rapture whose tremendous tension occasionally discharges itself in a flood of tears - now the pace quickens involuntarily, now it becomes slow; one is altogether beside oneself, with the distinct consciousness of subtle shudders and of one's skin creeping down to one's toes; depth of happiness in which even what is most painful and gloomy does not seem something opposite but rather conditioned, provoked, a necessary color in such a superabundance of light; an instinct for rhythmic relationships that arches over wide spaces of forms - length, the need for a rhythm with wide arches, is almost the measure of the force of inspiration, a kind of compensation for its pressure and tension.
Everything happens involuntarily in the highest degree but a in a gale of feeling of freedom, of absoluteness, of power, of divinity. - The involuntariness of image and metaphor is strangest of all; one no longer has any notion of what is an image or a metaphor; everything offers itself as the nearest, most obvious, simplest expression. It actually seems, to allude to something Zarathustra says, as if the things themselves approached and offered themselves as metaphors ("Here all things come caressingly to your discourse and flatter you; for they want to ride on your back. On every metaphor you ride to every truth - Here the words and wordshrines of all being open up before you; here all being wishes to become word, all becoming wishes to learn from you how to speak").
This is my experience of inspiration; I do not doubt that one has to go back thousands of years in order to find anyone who could say to me, "it is mine as well."