The self confidence you are looking for is a visible manifestation of high self esteem.
Self esteem is the confidence that you are able to be able to cope with the challenges in life. It is not over-confidence that can lead to arrogance or complacency. Self esteem is also the belief that you are worthy of happiness and acceptance by others.
Here are a couple of articles about self esteem that talk more about it.
One of the ways you can improve your self esteem is to use a mixture of visualization and affirmations.
You can visualize situations where you were able to talk about your knowledge and trust yourself to make decisions. Think about how you would feel and what it would mean to you.
As well as visualization, use affirmations to work on your self esteem. Come up with some short, positive statements like "I can make great decisions" or "my opinions are valid" and repeat them regularly, preferably 2 or 3 times a day. You should experiment with affirmations until you find some that you feel are right. Then you'll need to be patient - it can take a few weeks or maybe even longer before they really sink in.