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First: Write ALL your story ideas, one after another (not in detail) on a piece of paper.

Next: Pick one you want to write about and start mapping the plot. You can do it mentally or on paper, whichever comes easiest.

Then Start working on the events, characters, and details for the story. If you get another idea, write it on the Story Idea list and tell yourself that youll do it later. And maybe you will.

I do the same thing, and it's okay. Normally I find other ideas and I can go back and work on the story. I'm working on two or three right now. Sometimes you can combine stories into one. Maybe give that a go :) Sometimes it's impossible because they're about two different things, but I've been able to in certain cases. Just write from your heart and don't worry about it. If you don't follow what your heart is telling you, it probably won't turn out as great because you want to work on something else. But you definitely don't want to keep working on a piece unless your heart's into it. So just tuck it away in a safe place so that when you get new ideas, you can start working on it again :)