sketches aren't supposed to be good or bad. They are just getting the idea of something down. Too many people put pressure on sketches being amazing or perfect the first time. A sketch is not a drawing and a drawing is not a sketch. They are different things. A sketch can turn INTO a drawing, yes, but for me, that is very rare. Usually I will do many many sketches, revise the composition and draw it over and over until I get a sketch I like. Most of the first ones turn out to something I don't like though. That's the point of sketching, to work out your idea, not make a perfect image.
Sometimes it takes 5-10 sketches to get the right idea.