Hey Andrew, from the many want-to-be authors or writers I’ve talked with, this seems to be a common difficulty. People have so many ideas that it seems that one of these things happen:
They aren’t able to pick on of their ideas and just keep jumping from idea to idea.
They try to shove too many ideas into one story.
They start a story, stop it, and then start a new one (jumping from story to story, never finishing one, my sister is like that).
My only advice here is that it is of paramount importance to decide on the basic gist of a story idea, and then stick with it. Stick with it to the point that you ignore all ideas that you won’t use for that story specifically. Don’t even write them down. As a Stephen King once said (and I paraphrase) “if it doesn’t stay in your head, than the idea isn’t worth keeping.” (I don’t recommend reading King’s books, but he has some great advice)
I also have that ocean of ideas in my head, but once I decide on a story idea, I incorporate all the ideas that flow well with the story, and I either discard the rest, or tuck them away in my head for later. I then stick with that story until it is complete, and then I allow myself to think on the other story ideas.