Sayak Ghosal

It is because you are improving, self criticism comes when you want to improve more, for people who become satisfied with anything they made there are very little chance for them to improve, art have no set guidelines it is an constant trail and error process, so not limiting your self by being satisfied with what you know is a great motivation to improve. 

Drawing is an illusion, it is a process of portraying your observation of a 3 dimensional object into a 2 dimensional surface, art have always been a language of expressions and observation, and drawing is the grammer of that language, the better you are at Drawing it will be way more easier for you to depict your thoughts or observation making them a better art piece. 

(Drawing, Art and painting are not the same thing)

Pick a comfortable corner in a crowded area such as bus stand or rail station with a comfortable size sketch book, now be observant, try capturing the gesture of the people around you, at first do not focus on any details just capture gusture, next when you become comfortable now focus on proportions, when feel like you are able to do that comfortably focus on the contour lines. Convert the complicated shapes into easy shapes say circle, triangle or rectangle now add volume to those shapes. Refining those shapes with time will eventually make you an expert in anatomy