@Mommy_Connoisseur first off, thank you, truly. I appreciate anyone who reads even a single word and its messages like this that keep me writing in the first place. So thank you, I appreciate you bro.
Anyway, as for your question. There really is no right answer, everyone has a different process, and trust me, there are many times that I’ve just wanted to start a new story because I’ll come across a great idea. How I keep myself to one story at a time personally, is that when I get an idea and want to write it, that’s exactly what I’ll do. I have a OneNote literally filled with stories and plots. Probably around 20 of them. So I’ll continue to write out the plot in my OneNote and then eventually I’ll want to write the story I’m working on again. Don’t keep yourself from writing, just don’t publish it.
If you get sidetracked and want to write something different, then go ahead and do that, just don’t publish the story and instead take your time creating out all the characters and plot points and then you’ll be satisfied for long enough to keep writing whatever you’re currently on and then you can come back to it again, and when you do, everything that you created before will still be there for you to build upon.