Consider messing up your art materials as the first mark you make.
My mother took a watercolor class (I was a teenager) where the teacher told the students to flick a little paint and water across the page before they started, and that would get them over this idea that they were going to make a mistake. This is a great idea! Of course, with watercolor, you want to maintain the white of the page and try not to add opaque white, so perhaps it wouldn’t work in every case.
But if you have purchased a sketchbook to record your ideas, and put in on your shelf untouched instead, why not start by messing up all the pages? You can stain them with strong tea, sepia ink mixed with water, raw umber acrylic paint mixed with water. You can splat the color, splash it, smear it on with a paper towel, brush it on. You can do a few pages at a time and prop your sketchbook open in front of a fan so it doesn’t get mildewy. I knew a guy who would buy a sketchbook and throw it in the tub before (and sometimes after) working in it. The pages would fuse and morph, wrinkle and stain. Wonderful!
After you have done this, you will be intimately acquainted with every page of your sketchbook—it is YOURS. Now you can make the second mark!