Firstly, you should eliminate the negative people from your life who spoils your mood all the time. When people around you are self confident as well, this can boost your self confidence and make you a more positive person.
Just like training a skill, you should train your confidence. Try the things you have never tried before, and expand your comfort zone.
Change your belief system into positive. Don't blame your faults to anyone else or your past; you have the control of your life. Always try to find the good among the evil, and don't forget every failure is an experience.
Always believe in yourself, and try being the one who suppresses in a conversation. Here is a great example of how Ali supresses an interviewer both verbally and physically. After the match, interviewer tells Mohammed Ali "you are not pretty"
Ali's answer is: I am the king of the world; I am pretty. I shook up the world!
Act as an self confident person till you eventually become one. Our bodies govern how we feel about ourselves; just like animals, people tend to shrink when they have slim self confidence, and expand when they have high self confidence. These postures are called low power poses and high power poses. You must implement high power poses in your life.
I suggest this great Ted talk if you want to learn about it more, and here are the examples of high and low power poses: https://www.youtube.com/watch?v=Ks-_Mh1QhMc
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