When I first started experimenting with watercolor, those initial attempts felt like I was working blind! Oil and acrylic really do forgive mistakes - you can paint over them, scrub things out, change your mind mid-process. With watercolor, you don't get that luxury, and it creates a psychological barrier. But it's not just that, and it's not really about the one-way "light to dark" rule either - that's just a technical thing that's easy to learn.
The real thing is that watercolor demands a completely different level of control over the material. You have to not only visualize the result in your head before you start, but also understand how the paint will actually behave on wet paper, how fast it'll dry, how one shade will blend with another right on the canvas. Humidity, room temperature, the absorbency of a specific paper - all of it matters. That doesn't mean watercolor is harder, it just requires a different way of thinking! You need to be more decisive with your color choices, work a bit faster, not be afraid to leave gaps and white spaces. With oil and acrylic you can take your time and keep reworking; here you make a move and it's done!
The easiest thing that helped me was to not glob the paint on thick, but work with transparent layers, letting each one dry properly. And definitely find "your" paper - good quality paper changes everything!