I have so many character floating around that they all want their stories and their breaks, so when I start writing some on one story all the thoughts redirect to another story. It makes things challenging when I want to make a story seem to have more flow.
Is this because I don't write a plot line or outline for my stories prior to writing? Or is it because I let my characters live their own lives in my brain, doing as they please...
Does anyone have any tips for sticking to a single project at a time?

Oh man, I have the exact same problem. Honestly, what I do is write all my story ideas in a document and then try to work on a single story at the time. And if the story I'm working on really doesn't seem to be working out then I start something else. You also might want to try NaNoWriMo, National Novel Writing Month. National Novel Writing Month is their website. It really helps you to focus on a single story, although the quality does suffer a bit, but at least you get a good chunk of your story done, and you can edit it later.

I used to write the same way, honestly, and looking back, I suffered a lot because of it. I flew by the seat of my pants and abandoned stories when I realized how flawed they were. ._.; Some people don't need to do any planning, but most of us do. I think you should at least plot key events or make a time line of events so you can keep track of everything. I have far too many characters as well, but I grow attached to a story more if I plan every detail. Then again, that's just me, and I tend to multi-task to the point of insanity. I don't work on one story at a time. You might be like me and can do more than two at once if you plan everything, or if you grow frustrated with one of the other stories you can flip to the other for a while. However, that could also be why I rarely finish anything... Wow, I'm probably the wrong person to answer this...

I have the same problem! I have gotten better about it, but I'm really bad at planning my stories out because I like to wing it. I can plan out the whole story in my head but once I actually have to plan it out/put it in an outline, I notice how many things I left out or I don't know how to pull it all together.
What has helped me is to right down the summary for your story, what the main plot is, themes/conflicts, and who your characters are. These are good to reference to later. Try doing an outline too, it doesn't have to be anything grand and you don't have to follow it exactly...it is useful for writer's blocks. Hope this helps!

I also have the same problem. I have at least 27 original characters floating around. When I start writing I pick at least 4 or 5 of my characters to introduce into a story and then half way through I take a break and start something else. After a while I go back to them and realize how much the characters have grown since then and I delete them because the characters in the stories don't fit them as they have grown. Does any one know of any good writing promts that might help?

I'm an absolute slave to my inspiration sometimes. I'll start on one thing, then get another idea the next day and start on that... eventually I make a pile of scraps and unfinished things. But that's OK, I save them for later~