Anxiety has made a lot of people to underestimate what they can do especially during presentations which can reduce their academic performance while some people are using their full potential because of how confidence  they are at what they do even if they might not be the best.

How to be confident in what you do:

Be optimistic: Always have a good thought about yourself, you can be optimistic by having a daily mantra like I'm not a loser, I'm a winner.

Self awareness: For you to be confident you have to know your strength and your weakness i.e focus on what you are good at which can give you confidence while doing it.

Inhaling and exhaling: this can be used if you are nervous about what you are about to do, it can be used by taking a deep breath and exhaling slowly e.g I'm a shy guy that usually gets nervous before a presentation in class so I use the inhaling and exhaling method to calm down before going for presentations which really helped me a lot.

Posture: Having a good posture also helps in making someone to be confident i.e standing straight with your face up and your shoulders backwards is a good posture when you are presenting in places like class, workplace etc.

Note: It's good to be confident but don't be over confident