I've been trying to make a conlang of my own for 10 years now. And even now, I have NOTHING to show for it. Literally, nothing.
I can't decide what I want. My problem isn't 'oh, I learned about a new feature in some weird natlang, I need to add that!'. Problem is, nothing is perfect to me. Isolating languages lack information, agglutinating languages are too long, inflecting languages are too complicated. I can't get an vso order to work, svo is too cliche, sov is annoying to me. I want free word order, I don't want grammatical case. All I've managed to narrow down is I prefer simpler phoneme inventories. That's it.
I did make some progress in one conlang. I abandoned it for the time being though, because well I made it a limited language like Toki Pona. And no, mine isn't complete, for now I want to work on a more complete conlang. I did write down all I planned for the first conlang at least. I haven't done that in a while, record a conlang because I didn't want to even start writing until I knew what I wanted.
How do you stay with one conlang for any length of time? For me, my relentless perfectionism is what's always getting in the way. Nothing is good enough, and nothing can be good enough, but I can't accept anything less than perfection even knowing full well its futile.