I feel that way all the time, which is one of the reasons I don't write very often, haha. That, and no patience, but this is my advice.
Unoriginal stories with some "twists" and "tweaks" are still, well, unoriginal. At the same time, good luck coming up with anything that hasn't already been done in some way (not impossible, but very easy either). You can always follow some formulaic story, wizard takes a princess hostage, and some young person steps up to go find her. That gives you a frame, without any detail whatsoever, which is what's going to make your story. If you've got kickass writing and kickass characters, no one's gonna care if the idea's been done before, because you still wrote something worth reading.
Try to take what you want to "rip off," and cast it in a new light. Create a situation that's similar, but not so much that it's obvious where you got it from. Say you have a family of trolls or something in a story that come into town, massacre people, and steal a little girl. In your story, you could change it to be a warring state who invaded, introduced disease to the livestock, and stole some artifact (which I'm sure have probably been done before, ironically haha). But the idea is, you can take something you like and change it enough such that it doesn't seem like you're ripping anyone off, and you have plenty of leg room to make the situation what you want it to be to make your story unique.