For me it was advice from Dan Harmon: “Don’t try to prove you’re a good writer, you’ll never write anything. Try to prove you’re a bad writer and you’ll write everything.” Not perfect advice but it really does help me write when I’m being overly critical of my ideas.
Similar to the OP, the one thing I like to remember was a tumblr post from way back that boiled down to “write shit.” It doesn’t have to be good the first (or even third) time around. Allow yourself to write garbage. Look back at your drafts and half-baked stories sometime, see how far you’ve come. Work with the parts you like, acknowledge the mistakes and most of all, be able to laugh at the shit you’ve written. Can’t get any writing done if you set unreasonable standards for yourself.