For dealing with new story ideas/concepts, I just write them down in a file in case I need new ideas or I realize that I can twist it to make it fit with my current stories.
For focusing on finishing a story, I first have a general outline of where I want to take my story. Then I let my inspiration take me to what I want to write even if it means jumping into different chapters (Like today I could be writing for chapter 10 but tomorrow it can be chapter 5). When I get up to the chapter I edit it so the story flows well.
Another thing I do for current stories is to create a folder/bookmarks/playlist that gives inspiration to that story. I bookmark manga scenes, pictures, information related to the story. So that way when I lose the inspiration for that story, I go to that folder and playlist to regain the motivation to write that particular story.
Hopefully, this advice helped. Good Luck with your stories.

Um... idk how to tell you, but since I found a new writing style and understanding how I want my story to be read. It's easier. And I just go and write whatever in my mind then edit later. Or I go and edit my old stuff to get the creativity going. I guess it's matter in focus? But also what you usually do before you start writing, like what make you comfortable before you sit down to write, do that. Like I tend to go out downtown to get coffee and some cookie at the local shop before I go home, shower and do everything then sit and write. Or else I'd be on edge and not able to motivate myself.

So just do some stuff that is in your mind for the mean time first, then focus on whatever that is left over. If you have new plots and ideas, jot it down and leave it be and then go focus on your other stories.