I think first of all, you need to understand what you want to get out of your project. It's okay to just explore a thing that seems cool, and if that is all that you want to get out of it, it's okay to abandon that exploration when you're full.
I've gone through a lot of hobbies and projects, and each and every one of them has left me more capable. This is fine. I've also embarked on bigger projects that have taken years.
If you are working for some particular goal, sometimes you need to push yourself a bit too. It isn't always glamorous, you won't always feel inspired; that stuff does wear thin as you get farther along. Sometimes it is a slog, and those days just showing up and going through the motions is plenty. Sometimes breaks can be good too, but they have a habit of becoming indefinite so it's dangerous territory.
> This translates to my work as well, (at a startup) where my title is, quite literally, "generalist." I do stuff ranging from the software side all the way to marketing, sales, and everything in between. It's working for now but I get the feeling it's not viable for the long run.
It is absolutely fine to be a generalist. Having insight into all these areas makes you extremely versatile, and allows you to look at problems in ways no specialist could. Especially at a startup, this is an amazing skillset.