When I was in high school, I had a transformative English teacher who had inspirational quotes lining his classroom walls. One has stayed with me ever since.
Write Now. Revise Later.
That quote comes from none other than Harry Bauld, the former admissions officer at Brown University and assistant director of admissions at Columbia University (and current English teacher at NYC's Horace Mann School). His iconic book On Writing the College Application Essay saved me senior year and I highly recommend it for all my overachievers.
The hardest part about writing is getting out of your own head and silencing your inner critic. This quote helps remind you that all you can do now is word-vomit your first draft and then take time later to finesse it into perfection. (For best results, put your draft away for a few days so that you can revisit it with a fresh headspace and point of view for your revision stage).
Even professional writers feel pressure and anxiety to put pen to paper. Revising while writing is the fastest way to kill your creativity and multiply the pressure.
This quote should remind you that all you have to do now is write - it can be total, utter trash, but your only responsibility is to get your story on that paper (or Google Doc, if you will).
If you're still having trouble, try timing yourself so that you HAVE to finish your draft in the next 30 minutes. Turn off your computer's wifi and start typing. Don't finish until you've told your whole truth.