What you need is an idea notebook, a convenient repository to jot down your ideas as they come to you. Sounds like a no-brainer, but it does wonders. I keep a journal as a hobby, and I often write down my notes and thoughts on the writing process in it, but that's not enough. Matters of everyday life get written down as well; there's at least a complaint a day about how little sleep I'm getting.
With that in mind, I recently bought a small, plain blue notebook. Dedicate each page to one story idea. Write the rough idea, key sources of inspiration if you have them, and maybe a couple of possible titles to toss around. Now, if you're working on a story and it doesn't work out the way you'd hoped, you don't have to freak out about where your next idea is going to come from. Just tick a mark on that abandoned project, turn the page, and move on to the next one.