It’s often the case that the beginner artist is so determined to get everything ‘just right’ that they end up with a sketch that is stiff and overworked.
The problem is that they are so focused on the result that they are not understanding the importance of the process. The best and most characterful sketches have a relaxed and fluid feel, and this does not come with being pedantic; it comes from pencil miles. It’s that simple.
If you were to spend 30 minutes drawing one subject as carefully and as accurately as you can, but the person next to you spends five minutes each, on six rough sketches, who do you think has grown the most?
Mistakes hold little importance when you are learning to sketch. It’s all about exercising hand-eye coordination, and the more you do it, the quicker it will begin to flow. Accuracy will follow naturally, with practice.