Artists have two fears.
The biggest fear is color. That fear is pretty much universal across all mediums. The reason artists fear color is that our perceptual faculties are subject to many influences, making our ability to judge and compare colors unreliable.
For example, the "white" (or sclera) of the eye might actually be a dark brown under certain lighting conditions, but we would only be aware of this when we remove surrounding influences by isolating and measuring the "white" of the eye.
Artists trying to match a color are often surprised to discover that their assessment of a color isn't what they predicted.
The second biggest fear of an artist is a concern that they cannot judge the quality of their own work. They fear that even their best work is seen as bad by everyone else, and perhaps people are laughing at them behind their back.
Where does this fear come from?
This comes from the experience of observing other artists proudly displaying low-quality work, who seem unaware that their work is terrible.
The lack of awareness of one's own limitations is known as the Dunning-Kruger effect. It is defined as the tendency of people with low ability to overestimate their ability. In other words, they are oblivious to their poor results, and in fact, feel that they are capable, when in reality, they are not.
This leaves many artists wondering if they themselves are oblivious to deficiencies in their own work.
Overconfident people feel that there is little need to improve, and resist exercises that encourage artistic growth. I know a number of artists whose ego is greater than their ability, and they have indeed stopped improving.
If you are concerned that you might be among this group, then the best solution is to change your attitude and realize that there is always room to grow and improve. Maintain the view that you are, and always will be, a student.
If you want to improve, you must first become receptive towards improvement.