Paper…
It’s a weird thing that I’ve heard/seen/read other watercolor artists have… a near paranoia about “wasting” paper. For me, this comes a bit from the fact that watercolor as a media is transparent so once you make a (perceived) mistake, unlike other opaque medias, it’s hard to fix the issue. So one misstep and you’ve destroyed everything, (hence wasted paper).
You can layer things of course, and over time I’ve learned some ways to work with an unintended watercolor mark, but at the end of the day you either live with it, toss it (back to wasting paper), or take a deep breath, give your painting a second to be on its own, and comeback to find that it’s not ruined, it just took you in another direction.
Or you find tricks for getting over it. For me, I’ve found that making my own sketch book for some reason gets me past the mental block and paranoia of starting something, of that fear of failure and paper wasting. Yeah, I know that can sound weird… spending time making my own book with hand selected paper, to get over the fear of failure with that paper. But I find that crafting things from the start, accepting the uniqueness of the handcrafted sketch book gives me the freedom to accept the uniqueness of the sketches they hold, looking for those cool variations instead of thinking of them as imperfections.