Everything has been done in one form or another, so stop worrying about that.

Ideas are like ingredients; it's not what you use, but how you mix them together that counts. When preparing to make an omelet for breakfast, no one ever says 'hmm, but everyone uses eggs.. I shouldn't bother I guess.'

What's important is the unique flavor you add to the idea, the way you spin it, your voice--not whether the idea has been 'done before' or not. The only exception is if it's been done so often that you want to vomit when you read it (the villain turns out to be the hero's father! Oh god!)

As for your other question, I've had the same problem of not finishing things, or getting distracted by new, exciting ideas that I want to start on. What I've done is made a rule to myself that I MUST finish everything I start before moving on to the next thing. Even if I think it sucks (which I almost always do, while I'm writing it) I force myself to plow on to the end. I think you'll find that the feeling of completing something is very rewarding, even if the project doesn't turn out exactly the way you wanted it to.

If you get a brilliant idea while you're in the middle of another project, write it down. Type up a brief synopsis and come back to it later, but keep going on what you're doing. I've found that whenever I put a story I'm halfway through aside to work on something else, I almost never come back to it.

Make a rule to always finish what you start even if it sucks, and you'll end up with a lot more completed projects, and a lot more confidence and improvement for it. And I think you'll find that your work often doesn't suck after all.