But, in the never-ending quest to improve, expect to have frustration. But understand that that frustration is a vital part of making yourself better. It can't be forced. Learn to let that drawing go, and start on something else, even switching subject matter or medium. If watercolor is driving you insane, try oil, try marker, let your creativity explore!
This is why I use so many different kinds of sketchbooks. Some of them are low quality paper for experimentation and some, like my Moleskines, are for doing artwork that I intent to share with you guys. Though, let's be honest, even in my best sketchbook, I have pages I really think are horrible.
It's just a part of being a creative person. We have tons of ideas, are stimulated by the wonder of the world and life, and we want to record what inspires us in lead and ink. Always remember that your sketchbooks are not gold, not some precious gem or treasure. They are a tool to help you express yourself, to help you grow creatively.
Don't judge yourself too harshly when you don't like something you've drawn. Use that feeling to help you look at your art with a critical eye. Evaluate what it is you like and don't like, and how you can improve upon the next sketch.
Artists are notoriously hard on ourselves so learn to think about those little rectangles of paper and binding in a different light. They are your training routine, just like an athlete's. They workout, run, do laps, etc., to better their skills and we do the same on paper.
Visit your local art supplies store and feel many different types of paper. Let the sensation of the paper guide you and inspire you. Most of all, let it lead you to the creation of awesome art!