It isn't just you.
Storytelling is an old art. (Anyone with a need to look that up could let us know just how old.) When you worry that your newest mind-blowing twist has been seen before, it's probably not for nothing. The same goes for themes and character traits. Even Grendel's mother can't be credited as the first character ever to lose her life in seeking revenge for her child.
If you feel that a plot-without-precedent is your most important story element, then go ahead and remain paranoid, remain vigilant, and be aware that you might never complete a thing until (if ever) you've hit upon what only you could've plotted. That parenthetical is not entirely cynical advice. A writer who encourages another writer to abandon the search for originality is doing no one a favor. You might very well achieve that perfect, original plot one day. Keep in mind, though, that even a popular plotter like Dan Brown wrote less than popular, less than unique plots when he began. And what lies behind his plots-- conspiracy, adventure, puzzles-- is not new.
On the other hand, when your compulsion to monitor newsfeeds makes you write faster every time an author’s new publication or, worse, real life begins to mirror what you’ve been planning; when you look over your shoulder because current events are catching up with the plot you built from nothing but imagination; when you have to get the story out there before “out there” beats you to it... well, that's the groundbreaking plot you seek. Maybe.
Because once you get it out, of course, tons of your readers will blog, “It was great and fresh. Reminds me exactly of that novel by...” Sigh.
In the end, how much we stress out toward the aim of blazing a trail is up to us. Throw out every piece you write the moment you begin to worry about it, if you want, but know that you aren't assured to ever complete a title; expect very little recognition for how fantastically original your work is. Or you can writer-up and work through the doubt. Readers don’t require fantastic originality. Readers require good stories.
(By the way: I think you can do it. When you do come up with that most original of plots, let everyone know. We'll be sure to imitate it for you.)