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.
I don’t recall the specific wording, but the best piece of advice for me was something like this:
Don’t try to be a writer. Don’t try to be an author. Just write.
Having an ideal identity which I was constantly and very unfairly measuring myself against prevented me from writing anything at all because before I even got started I knew it wasn’t going to be to the standards of my imaginary avatar. I now allow myself to write a mess because in that mess is some quality stuff which I can extract and expand on, and writing the mess is a lot of fun. I’m currently totally free of an audience I have to conform to, so my writing is totally free from any kind of restrictions other than what I prefer at the moment.
I agree with this. I used to be unable to write anything because I felt like, if my work wasn’t read, it was useless and needlessly written. Now, I love writing and I’m able to do it every afternoon, it’s quite relaxing.