The internet makes you realize a lot of things, and I just noticed another one. Even on art sites most things you post will look very unoriginal. You drew a picture of a dragon? Nice, so did ten thousand other artists. It's really discouraging to see something you put a lot of thought and care into something just to have it turn out to be going through the same pattern as a hundred other things. Copying another artist doesn't make one look very creative and often sheds a negative light on oneself. One can accidentally make something that looks like a copy of something else (this doesn't mean I support tracing and claiming). And we all want our work to stand out. This creates a strong self expectation to be original. This may sound like a good thing, and I would agree it's mostly good. However I've often found myself focusing on making something totally unlike anything else rather than making something look good.

This leads to bad practices like trying to give my fantasy land a whole new set of words to distinguish it from other fantasy worlds. At one point I was seriously considering making up a new word for “year” just to distinguish my story world.

Fleshing out one's fictional universe isn't necessarily a bad thing, but it can easily cause one to miss the forest for the trees. Quality of one's work should not be sacrificed for originality.

Using the past ideas of shouldn't be carefully avoided either. Many great works were using other for a template. “Lion King” took its whole plot from “Hamlet”, and it was good. Not only is making ideas from the past ideas of other acceptable, I would argue creating ideas without doing so is impossible. All ideas use other ideas in some way. Mixing ideas is how new ideas are made.

Do not worry if your work resembles other ideas, rather worry about whether or not an idea fits well in your work. Don't make something original for the sake of originality itself. Make something that you love with quality.