This is a big question, but I'll try to address what I'm able to as someone who produced (mostly) goth/industrial music for around a decade.
My initial motivating factor as a teenager was simple: I liked listening to music, I thought being in a band would be cool (it is), and I knew how to play the piano, so I played with a few bands from my high-school, none of which I fit into very well creatively.
I liked goth music, industrial, EBM, etc., and a friend introduced me to the production of electronic music via a cracked copy of FL Studio, which set me off to the races. I would describe myself as more technically-inclined than creative, so all of my early music was inspired by the films and books I liked; it requires less imagination in my opinion to map out one piece of media in a different format (i.e. writing a song about the book Neuromancer).
In terms of my personal motivating factors as I progressed along my journey, I can think of a few things, some more noble than others perhaps:
1. The drive toward technical mastery
2. Recognition of technical mastery
3. Goth girls
4. Fans
5. Goth girls who were fans
In practical terms, the primary steps I took toward creating music involved a lot of work at my terrible teenage job to save up for gear, taking singing lessons, and studying music theory, followed by networking with other artists and promoters on social media (which was new when I started). I was extremely lucky and met an independent promoter/booker who shilled me relentlessly to everyone she knew, which set me on a positive trajectory.
After gaining enough momentum to start filling in for other similar but larger bands and opening a few shows, I had to get serious and start really studying music production (a professional engineer from a band I worked with offered me invaluable assistance with my first album on this subject). Perhaps most pertinent to your question, I studied other people's creativity. Do you want to write? Read your favorite author. Take notes. What do they write about? Which themes do they utilize? How do they structure their sentences? Take them apart piece by piece, then substitute something that resonates with you instead. Do your favorite writer's characters always die in the end? Should yours? Does something else make more sense to you instinctively?
You've probably observed that in my case, other people drove me forward. One thing I would recommend in the event that you start creating, is to try not to be too embarrassed to share your output, or reach out for help in suitable venues. A lot of people are jerks, but a lot of people will appreciate your contributions and help you out. There are many artists from every medium on here, for example, and all of them understand what it's like to struggle the way you are.
Motivation while suffering from serious depression and anhedonia is a difficult square to circle, and I wish I had some kind of brilliant advice that would part the heavens and disperse the fog for you. I've been depressed most of my life, but I was fully functional until a really crushing bout around a decade ago, well after I quit music (bad experience with a label). If someone asked me to walk into a studio and knock out another album in my current condition, I would die of laughter.
The question of finding fulfillment is also difficult to answer due to its subjectivity, but can you remember a time before the depression and anhedonia? What did you like? Your meaning doesn't have to be feeding the world or painting the Sistine Chapel. You're allowed to feel fulfilled by watching old movies, or playing video games, or fighting that one duck at the park who steals the other ducks' bread. Maybe just take it up a notch. Why do you like that movie? What are its underlying themes? Why is that video game's play loop so satisfying? What makes it so immersive?
This my longest post ever and probably mostly nonsense, but I love it when people create, whether they're creative or technical, Mozart or fingerpainting turkeys. I have to go lie down now.
