The major thing, if I had to pare it down to one major thing, that motivated me to do my first and best-known work as an independent creator was this: I wanted to write a story about an incubus with a positive spin that didn't paint him as monstrous or unattractive, because pretty much each and every story I'd ever seen up to that point did that. I combined it with my love of satire and irreverent humor, my hatred of censorship, and my irritation with stories where pretty people have great relations and somehow still manage to be miserable about that and everything else.
Ultimately we're the main people who should believe in what we're doing, or else why do it? It's not like we get fabulous bonuses from this career, unfortunately, in most cases. But some of the benefits make it very much worth it, like touching the life of another and changing it for the better. Or giving them some feeling like they're not alone, or that it's okay to have interests they think of as strange...because of their circumstances, surroundings, or upbringing. We can show others that what they've been told and exposed to is not the whole universe. There's much more to it than that.
In fact, I'm still doing comics even though that first solo series ended. I even had several false starts that didn't lead to anything in particular before deciding on Demoniac Verse, but I don't regret that experience. I learned, I developed, I still managed to touch lives, even if I didn't accomplish my desired goals with those projects. And even if I ended up moving on and putting them aside, they still helped me to come to a place where I could believe in what I have ended up doing with Demoniac Verse.
And as for it, I believe in it because there just aren't any comics I know with characters the same style as mine -- most aren't even similar -- or the same sort of dark fantasy approach tinged with humor, horror, sexy stuff, and perhaps above all, at least for me...gay characters who aren't treated either like OH MY GOD YOU'RE GAY or alternatively, a barge of medical waste. They just...are what they are. And the characters who aren't gay are who and what they are. I fancy it a mature approach to orientation, to sex and relationships, and other things, and I like to think that at least some readers are empowered and encouraged by this presentation. So it makes me believe more in what I do.
I also really love iyashi-kei or "healing genre" stories, so I try to incorporate elements of a healing feel in my own stories from time to time. I like to help people feel better by enjoying my work.
Basically the biggest thing that keeps me believing is that I have an audience cheered by my work who enjoy what I do. I'm glad of that and grateful for it. And as @AnnaLandin so rightly said, fanworks are like playing with someone else's toys; I'd rather tell my own story, even if it might have similar characters. And even if they're close, I can make a commentary or analysis out of it rather than simply celebrating someone else's creative expression. Either way, I feel putting in the work to develop my own world, setting, and characters to populate it, is worth doing for my own satisfaction, to express my own perspective rather than simply bringing my perspective to someone else's who -- in most cases in fanworks -- has simply cultivated an idea anticipated to be likely profitable.