Confidence can be hard to come by, especially when you need to do something that scares you. An easy way to drudge some up is to prove to yourself that you're smart by talking about something you know really well.
Photo by John Lambert Pearson
If you have a really important presentation, you're going to ask for a raise, you need to convince someone to help you out with something important, or whatever it may be, your nerves can get the best of you and ruin the entire thing. This is because you're terrible at predicting the future and are easily overwhelmed by fear and negative possibilities. Essentially, you're priming your brain to put yourself into a mindset that will cause you to choke, making the outcome certain and inevitable. Assuming you'd prefer this doesn't happen, you can quickly correct the problem by having a little conversation with yourself.
In the same way you end up a slave to your nerves by thinking about a worst-case scenario, you can regain your confidence by bringing up a topic you know well. Take that subject you know well and explain it to someone. If nobody's around to listen, explain it to yourself in the mirror. Ask yourself questions about the topic and offer up relevant opinions. The more you talk about something you know and feel comfortable with, the more comfortable you'll feel in general. Doing this will shift your mindset to a place of confidence rather than a place where you're a nervous wreck, and you can use that confidence to help you accomplish whatever you want.