I recently heard from a former student of mine. She tried doing NaNoWriMo - National Novel Writing Month, where people attempt to write 50,000 words in November. She hadn't made the word count. She lost, and she was feeling really discouraged by this.
Here's the thing, though. If you attempted to but did not write 50,000 words in November, then yes, you lost NaNo. That doesn't mean you're not a writer, or that you can't finish a book. The only thing that proves you can't finish a book is not ever finishing one.
Starting a book is great. It's one of the best feelings ever. Here you are, with your shiny new idea, and your awesome characters and you know how this begins and you sit down and writing is just So. Much. Fun.
At some point, it won't be. At some point, you will wonder if perhaps your characters are in a different book than you are. At some point, you will be convinced that every single idea you have is flat, stale, weary, and unprofitable, and then you will hate yourself even more for phrasing it in those words because Shakespeare already expressed despair perfectly and why are you even trying, why? At some point, you will not know what happens next. You will tell yourself your book is irredeemably flawed. Perhaps you will stop writing.
That is how you lose.
At least if you want to be a writer. (Maybe you learn that you don't want to be a writer. That's not losing. That's increasing your own self-awareness. That's good.) But if you want to be a writer, the sort of writer that publishes things, you cannot just write the first third of a novel, and then put it away when things get tough. Things are always going to get tough. Welcome to the Dreaded Middle, my friends. It's not a nice place. Even the Fire Swamp is more fun. But the only way out is to write.
You can always fix a flawed book once it is written. You can't fix what doesn't exist.
So maybe you didn't write 50,000 words last month. That's fine. Unless you're under contract, no one cares when you get your book finished. Keep writing. Finish it. Otherwise, you never know what you could have done.