Those uber confident people you see and think 'I wish I was like that': the vast majority are faking it! Confidence is a mask, an act, a trick.
Famous actors feel sick and puke before going on stage. University professor shake with nerves before a lecturer (podiums are there to grip on to). Teachers dread new classes every year. Etc etc etc. But they appear cool, calm and confident to others. It's other's perceptions that count, not how we're actually feeling.
You can learn to SEEM confident much more easily than learning to BE confident. Often the latter will happen anyway if you do the former and realise people buy into you as a confident person.
Body language, posture, eye contact (fake it by looking at the eyebrows not the eyes. Lol) are key. If you fidget, always hold a hardback notebook with both hands. There are hundreds of tricks, websites devoted to it. Practice them in the mirror and find which works best for you.
People have said they wish they had my confidence, that no one scares me - they should feel my shaking hands, my acid relux, my sweaty palms, the time I spend deciding what to say and then over analysing what I actually said. But I walk into a classroom of thirty 17 years olds looking like I kick ass and know what I'm taking about.