In human species we find a wide variation in personality which plays a vital role in our progress in career. Vast majority of us are unconscious about it as it falls in the "Knows not knows not domain". Anything falling in this domain may not be known to us. What we are not aware of can never be acted upon. Although there are several theories about the causes for variation in personality, I am in favor of 'Psychological conditioning'. This means recording a message in the subconscious by repeated saying by others or ourselves.
Subconscious is the most powerful part of us which regulates our behavior upon which we have no control. It is like the operating system (OS) of a computer embedded in the chips by the programmer. Like the computer functions as per this OS we act as per the subconscious.
Ever since we have born we are constantly in conversation within us. Only when you learn meditation and practice for a considerable period of time, you will not be able to recognize the presence of mind as a separate entity as an instrument for your usage. For common man who is unaware of this aspect, the mind decides his state of being. If the mind is happy, he is happy; if it tense, he is tense!
One of the conditioning is happening through what we talk to ourself. 24x7 all of us are unconsciously talking to ourselves invariably negatively such as "I am not good at this & that". One of the famous book written by Shad Helmstetter titled as "What to say when you talk to yourself" deals with the impact of self-talk and suggestions for positive assertions. Please read this book and initiate "auto suggestions" to make a big shift in your personality.
With whole of my heart