"Where is my motivation to keep going?"
It's where it always was before fanfic, before online support; before recs, before asks, before moots, before fanart.
It's in realizing you're the story's only way out into the world.
In a world full of gatekeeping, this is the gate that only you keep. Turn your back on the responsibility to open the portal to the unborn (original) story and keep it open, and the story dies. And that death is on you.
Yes, it's lonely work, without the constant rush of input we've been trained to be used to. It's been lonely work for a long time: since the first storyteller came up against the silence that wanted to keep the story away from the breath that would make it real in other people's ears. And you could make a case that all the online adornments are just our recent generations' way of keeping both the storytellers and the listeners from being overwhelmed by that loneliness. (Because the listeners have their own version of it: the fear of what happens when the people who can tell stories fall silent. Good storytellers respect that fear, and remember every day their responsibility to do something about it.)
Where do the characters come from? A surprising amount of the time, without warning, they muscle their way into the back of your brain and grab you by the hand (or hair) (or throat) and shout Tell about me! You have to tell them, there's no one else who can do it! ...Sometimes you have to sneak up on them from behind, as you do get the shy ones occasionally whom you have to take by the hand and pull into the light. But give them enough silence—build the space for them—and they'll come.
The silence may be key. One of the smartest pieces of advice I was ever given was that, for half an hour in the morning every day, before starting work, I should sit down and do nothing, and listen. No music, no TV, no news, no reading, no nothing. Sit and listen. It's not meditation; it's not mindfulness. It's listening. Story's voice can be hard to hear, sometimes, until you get better at pushing aside all that relentless rush of situational and sensorial input, and better at waiting to hear the story that's as yet too frail to push its way through the portal without assistance.
To be clear: Fanfic work (or any work in universes not of your making) is a different kind of listening. Working well in already-extant universes requires sharp attention to the tones, concerns and qualities of voices already speaking there; and to a certain extent, to the voices speaking about them. And if you love the characters, too—one of the best reasons for fanfic, really—that's a pleasure.
But when working in your own universes, the listening also requires a selective quality, as the characters find their voices and their proper passions. And as for the love... you're the only one there is to love them, till you get them out into the world. If you've ever been the only one to love somebody, you know how tough that can be.
Then add to that the fillip that those people (or situations) won't be really real until you've worked with them long enough, hard enough, all by yourself? It's a tough row to hoe. And you can't ever be really sure that a summer will come to reveal whether the crop's taken root, and whether it's all been worthwhile.
Nonetheless: it's good work. Some of us don't seem able to stop. Some of us even like it that way.
When you're ready, take that leap and come join us.
