The good news is that we can change this! Trusting yourself is a habit and could be developed through repetition! Since we feel unsure in 2 cases:
1) when we experience something for the first time, we deal with something unknown.
2) when we experienced something negative previously and have to repeat similar experience.
For example, when you have to talk in front of the audience, your emotions defined by previous experiences. If you didn't had the supportive audience before - you will feel very insecure or even stressed. Have you ever noticed that people, that have been supported by their family in childhood, feels more self confident and sure when they talk about something or take actions? It's because they have positive experience and are not afraid to fail.
So, how you can develop trust in yourself?
1) Practice more then 5 times things that you feel unsure ( for example, your speech in the meeting)
2) Surround yourself with people that support you and believe that you will be able to succeed!
3) Define your threat and take every day before one of the meals a little action that make you step through this fear! Develop a habit, that increase your trust in yourself!
4) Write down your successes in a journal and re-read it in the end of the month!
5) Every time you notice an evident success through your fear - celebrate it and fix the progress!