I asked a question almost exactly like this (http://ca.answers.yahoo.com/question/index;_ylt=Av7Ot9UIpOjOHKD2jkzERvzAFQx.;_ylv=3?qid=20080411134733AAxSbdw)
So I can relate to your issues. Any time you find an idea that seems very close to another concept, tweak it. Remove certain elements or add some of your own. And to be a writer is essentially to be a thief. There isn't a single accomplished author who hasn't nicked some ideas from other people. Don't worry too much about it if it's minor.
Try not to revise. If you have an idea that you really like, that you think is better than the original one, then put it on trial. Weigh it. See how it could connect with other plot elements.
In the book I'm currently writing, I have a rather major cliché. I played with that cliché a while back, and as I weighed it against the plot I had, it fit like the missing piece to a puzzle, and about five loose ends tied together. I don't worry about it anymore (Try not to use clichés though. it usually subtracts to the experience).
With regards to the craft of writing itself, books are your best friends. There are half a billion books on how to write, so thinking you're not good enough at it is no excuse. Teach yourself. Read books in the genre you want to write as well, and take some cues from the author.
With regards to inspiration, music is golden. Listen to it when you write (if you can focus still) and you'll get some good stuff. In fact, about three minutes ago, the chorus of the song I was listening to gave me a nice big wad of ideas that I think will work really well
Lastly, if anyone tries to put you off, just laugh at them and get back to your writing.
Good luck!
~Tiger