I tell students to write rubbish. Always. And quite a lot of the time they look a bit shocked. But I mean it. Write rubbish.
I tell students to write rubbish because the worst bit of rubbishy writing on the page is worth more than the most perfect bit of prose stuck in your head. Stuff on the page can be improved, developed, tweaked,given colour and life and energy and style. Stuff in your head is - well, stuff in your head. It can't be read by anyone.
Give yourself permission to write badly. Accept you'll have to re-write - and I don't think there can be any professional writer who doesn't consider re-writing as part of their process. It's what we all do.
An agent won't read your work with more interest because it appeared fully formed on the page. An editor won't clap their hands in delight because you wrote in a linear way, starting at the beginning and working your way through until you get to The End. A reader couldn't care less if you didn't need to use the spellchecker.
All that anyone cares about is the finished product. How you get there is up to you. Write rubbish, if it gets you writing. Write, write, write. That's all that matters. And then you can make it better.