As an early-career writer you work about 90% from the heart and about 10% from the head. Stuff pours out of you. You can lose track of time while you're working. Everything's inspiration and there's little in the way of calculation. I've heard writers claiming that they go into a trance-like state when they work and, frankly, they're right up their own arses; all they're trying to do is impose some mystic significance on that undisciplined rush.
To be carried along by that kind of inspiration is an exhilarating experience but, inevitably, there's a downside. The heart is not a great organiser. The evidence is there in all those would-be debut novelists who can't muster the craft to place a single short story with an indie magazine, but keep on sending out their 250,000 word epic.
Being a pro means being in control of your gift. The same's true whether we're talking about writing, art, dance, sports... anything. An outpouring of the heart can be high-value stuff but, like a kid in a kitchen, it needs a hell of a lot of cleanup to get anything presentable from it.
And when you're starting out, that's exactly what you do. You pour it all down onto the page, and then you look at it. You think it's wonderful. Others don't seem to get it. What will it take to make them see it the way you want them to? What do you have to change? What do you need to lose? What do you have to supply that isn't there already? Do it again. Do it different. Do it better. Make it work.
The balance starts to shift as you instinctively seek that magically right proportion of feeling and thinking. You learn to recognise what's right before you set it down, as opposed to having to work out what's wrong afterwards. If the balance shifts too far, if you become all calculation and no inspiration, then you fall into the habits of a hack. But mostly the process seems to take care of itself over time, as long as you keep at it.
I've lifted these thoughts from the afterword to the Telos Classics edition of Valley of Lights, in which I ramble muchly about the background to the writing of the novel and the circumstances surrounding its publication.
I may put more of it online in time, if I can work out how avoid dumping one big TLDR on you.