I agree with Victoriavictrix on the writing thing. I had a wonderful original fiction idea for publishing, came up with it all by myself, and by accident stumbled upon a previously published work that carried many of the same themes as my book. I didn't let it discourage me from writing out my ideas though, I told myself "I'll make mine better, I'll make it
mine." And many fanfiction writers, you have to remember, are already writing from the vantage point of characters that are not their own. They fully realize this, and thus, why it is called FANfiction.
My original works aren't derived directly from things I see. You won't see any direct carbon copy works with character names changed. There's no challenge in that, no fun. I like superhero stories. It doesn't mean I'll be writing about "Catman, hissing avenger of Cotham City" any time soon. I twist and build my stories like a sculptor sculpting clay.
If a writer writes the exact same story as you, that is called
plagiarism and it falls under a completely different category from similar ideas. I could tick off a list of at least twelve "Angel of Death" stories (even ones who may or may not be), off the top of my head. Does this make your story any less valid to you, or the people who read it? Not at all, if anything, the better you do something, the more it's
your story that will stick in their minds.
The difference is in the details, and the details are entirely your own. I wouldn't worry so much about people stealing ideas from you, because even if they stole an idea, it's still just an idea, not your story. They're going to have to build their OWN work around that theme, and like victoriavictrix said, no two authors are going to write the same story. Part of the writing process is feedback and criticism, and to get both you need others to actually read your work.
DollyKim, you strike me as strong enough to stand up to anyone trying to bully you into something you feel isn't true to your characters.
Besides, and please don't take this the wrong way because I mean no offense, but you just said so yourself that one of your ideas came from another person's story. That's essentially saying "I can do it, but I don't want anyone to do it to me." (That sounds so condescending but I have no idea how else to put that, I'm so sorry, please don't think I'm standing on a soapbox here.)
If anything, just remember publishing a book professionally you have to take into account that other people are reading it too, and there has to be some give-and-take between what you want to write and what readers want to read. If you want to write something fully for yourself with no outside influence whatsoever, do what I do and just leave it on your hard drive to enjoy by yourself.
Otherwise...concessions do have to be made, no matter how much an author might love to bypass editors and rewriting suggestions. (Wouldn't we all? X3)
In short: Please don't take this as a criticism of your character of any kind, just a reminder since you mentioned you wanted to eventually professionally publish. I mean no disrespect, nor ill intent, if anything it is the opposite and I would love to see your book sitting on a shelf one day.