I have the same problem too. I mean about not being able to finish my stories. Now I just write all my ideas down but continue the story, keeping them fairly short. When I finish a story, I see the ideas I've written down and pick one. That's how I like to do it.

About the Mary-sues. I also hate all-powerful characters. It's always annoying to me how sues are always the evil person. Well, it makes sense but can't the good people be all-mighty for once? But it's weird how the Mary sue always gets defeated at the end. That means they have to have a weakness, just the readers don't know that. I would appreciate it if the author put the weakness in the story once in a while.

I hope I could have been of help!