In your brain, it’s like a movie. Your characters are 5d. Your settings are magnificent. The dialogue is witty and bright and cinematic. On the page it’s more like garbage. The characters are inauthentic, their words are flat. You know what the room they stand in looks like, but you have a feeling your readers won’t. Don’t panic and don’t give up. First, you might need a break before you tear your hair out. Take a walk, journal or write something different, watch a favorite movie, read a favorite book or listen to an album that transports you somewhere. Bonus points if it expresses what you’re trying to express.
Now understand: you will not always read your writing and see how brilliant it is. You wrote it! Of course sometimes you think it’s shit! (but of course sometimes you think it’s fantastic). But keep going even when the words on the page are imperfect. You will revise it later. Your job is not to write the perfect version of your story on the first try, or even the third try. Your job is to write.