Keeping focus is one of my biggest problems.
trying to keep ideas separate is so hard the method I have is to work on 3 or 4 ideas at a time but this is probably a bad idea cause i never seem to get anything finished.
The way an idea proggresses and changes as you go is all part of the creative process and is somthing that seems to happen in all forms of art you start with one idea or mood and the longer you spend on it the more you want it to evolve in to somthing new(a bit like a set would if you like)

But I allways seem to loose the original idea/mood I was trying to get accross and end up with somthing compleatly different

Its all part of the fun I guess.
Its good to know im not the onely one who has this problem or maybe its not realy a problem and realy its a blessing.