The impulse to make marks is as natural to humans as the impulse to talk. Babies spend their first year or so making strange sounds, imitating what they hear. Through practice and interaction, copying people around them, they learn to make the sounds that others understand. Without this vital preparation they would not be able to communicate. Children are then taught to read and write, again making mistakes and slowly learning the basics. .
Only when they are comfortable and confident with the rules of grammar, composition and writing techniques can they take the next step; that of creating an original piece of writing. To do this they need to reach out for raw material; something which inspires them to write and to want to share their thoughts.
Learning to draw is similar to learning to speak in that it is a perfectly natural thing to do, but for some reason we don't think it should be a slow process of making mistakes and learning through practice. We seem to have an unrealistic expectation of being able to learn a few basic techniques and then turn out masterpieces for everyone to admire.
Do yourself a favour - don’t expect to turn out ‘proper’ drawings when you are doing exercises. They are designed to help you learn to SEE and are steps to being able to draw well. Your progress will show in how differently you start to see things around you, not necessarily in the drawings themselves.......
Never be afraid to make mistakes; they will teach you much more than anything else.
“The sooner you make the first five thousand mistakes, the sooner you will be able to correct them” Kimon Nicolaides 'The Natural Way to Draw'
It all depends on you, and how much you are prepared to invest in practice.