"There are no original stories/concepts left," someone says.
"But they haven't been told/covered by you," another replies.
We know this exchange between writers. One person states the first line, often as a lament, while the other leans on their crutches in determination (or cheer) and recites the second.
Once the battle is won within the self, the writer begins the work for themselves (and their readers). Then, the waiting public can start the doubts all over again.
Minority authors (and women) often face the Already Been Done (ABD) criticism from many types of people interacting with their work, regardless of how unique the concept is or how beautiful the prose. A book about a disabled artist? It's been written. Coming out as homosexual in the Bible Belt of America? Got it covered! Yet another cancer story? We have enough.
If one well-known piece of literature exists pertaining to a struggle or life of a minority person, people consider it Already Been Done and discard it as such. A lot of the same people dismissive of these stories will generally applaud another coming-of-age story about a (cishet, white, neurotypical, etc.) guy. Why is one "unimaginative and derivative" but the other is "paramount literature"?
It stops people from telling the stories they want to tell, from books and essays getting to the readers who need to know they aren't alone or that they're valuable enough to write about. It also gives everyone less variety to choose from.
If you receive feedback that your work has been written before:
1. Consider the titles the person lists in comparison. Are they recent? Do they have your slant, or just your topic? How is their writing style? Analyze.
This will (mostly) put your fears to rest.
2. Consider the source. Do they have varied tastes? Are they outright dismissive of you and/or your work (maybe a bit cruel)? How many times have you been told this, just once by this person or by five different people? What is their background?
Always find multiple people to give you opinions, never let one person deflate the passion you possess. If nearly everyone you trust tells you that your story is too much like another, examine it then. Maybe you just aren't taking the right slant or you're distancing yourself from the hard parts.
Remember: There are no truly original ideas anymore, but there are original "voices". Use yours.