I would say, adopt a discovery writing paradigm, and focus on a character. Most of my stories begin with a character that has some rare (and interesting) real-world ability. I find a matching thing she sucks at it. I pick an age and start writing about her, deciding things about her, maybe she has a love life, maybe she doesn't, maybe she has a sex life, maybe she doesn't. Who are her friends? How did they meet?
I am really writing about her, I'm not talking about notes or a profile. I am making scenes for her, in class, or at work, or at home, on a date. I don't care if I keep them, in the end I usually am not keeping the first several scenes I wrote. I want to give her a problem and see how she solves it. I rewrite these scenes several times, trying to make them better. Then I may discard them and come up with a different problem for her to solve. Heck, I've even changed her setting, from modern to medieval!
But she is ONE character, my main character, and all the other characters are peripheral, and this story is about her, and in some way (even if she is forty) a story of how her life was transformed in some way. How she discovered she had power, or graduated college and found her place in the working world, how she met and married a spouse, how she learned she was gay, or perhaps in college how she decided to become a lawyer or doctor.
In the beginning she is malleable, I can give her strengths or weaknesses, or take them away, or reverse them. She's good at math. Nah, she sucks at math, but she won't let that stop her! Or she's capable of self-defense, and I want her to prove it, so I have a scene for that.
These scenes can be disjoint, forget about a plot. Pick scenes from her past, maybe even scenes from her future; long after the story setting. Write to show her character, do not write a character outline with a bunch of dry telling about who she is, write scenes that show who she is. Write anything that occurs to you, if it occurs to you then it likely serves some purpose in your creative process of delineating her character. You can always toss it if you don't want anybody else to read it!
For me, the advantage of this approach is that I fall in love with my character. That's it. If I write enough about her, and think of enough scenes, sooner or later I come up with the "big problem" she is about to face in her life. After that, I can choose the setting and her age and I am writing about my MC in her normal world (easy after all that exercise, although it might involve a bit of research), and we start in on her discovering her big problem, whatever is disrupting her normal world that she must do something about. This could be saving her world, or just she's lonely and sick of it.
I don't plot, exactly. My character does what she does, always facing some difficulty or another (sometimes of her own making), and I know her and I am getting to know her better, as she navigates these hurdles.
I DO typically have a resolution for her big problem in mind, I write a sketch of that (not prose, just how the final scenes will go). I am not married to it, except that if my character does anything that makes that ending no longer viable, I have to come up with a new and better ending, or I have to reverse course in my writing and undo the scene or scenes until I get back to a place where I can plausibly have her make a different choice that doesn't kill the ending. 3 times out of 4, I can think of a better ending and let my girl have her way, with little or not rewriting. I prefer that.
Much of what you write makes me think you should be a discovery writer, not a plotter.
The reason I am a discovery writer is because plotting a story ruins it for me; it feels like I have already told it, and when I am writing it feels like a straight-jacket, I can't do what seems like the obvious thing my character would do, and then she feels artificial and puppetized. I can't stand it.
I discover the story as I go, and if I think of something brilliant, I will think about it, figure out where it would go, and either salvage or scrap what I need to tell a more brilliant story. I suggest frequent time-stamped and separate backups; mine are backup up hourly (if their last-written time has changed).
The point here is to give yourself an anchor. You are writing a story about this one character you have imagined and developed. Don't worry about the plot points so much, don't worry about the villain (if there even is one), just give yourself a character that feels real to you, with a more-or-less inevitable life-change coming that feels real to you, and keep writing that character, and giving them obstacles, and do what feels right to you for this character you developed.