I am confident.
I wouldn't necessarily say that I believe what I say or my thoughts have value though.
Value is something someone/society put on something. Take dollar bills for example, there are all pieces of paper, if we didn't decide that they had any value, they would be worth nothing (in fact during quarantine where paying in cash was restricted it very much felt like they were worth nothing), there is nothing that technically justify a 50 dollar bill (piece of paper) being worth more than a 1 dollar bill, they probably costed the same price to make we just decided one was worth more than the other.
I think my thoughts and what I say are valuable to me but they are as valuable to others as other find value in it.
My confidence isn't affected by how people perceive or don't perceive what I say or think though, which I think is where true confidence come from.
If you go to an interview and think it's YOU they are judging rather than if you would suit the position, and walk out of an interview self-conscious and deflated, you are looking at it wrong. Most people, for the most part, don't care nor give much value to others and their thoughts/what they say, nobody care that much and definitely nobody care about you half as much as you. Most of the time, people's behavior/lack of interest isn't personal, when people reject you, it's very rarely YOU they reject but more so the fact that you might fit within the pre-made vision they had of what they feel they need or are looking for. It's very rarely about you.
I think the key is in knowing your strength, acknowledging your weaknesses (and working on them) and going for what you want unapologetically, knowing you aren't perfect but nobody is and that what people think of you doesn't take away from who you are.
When I go to an even some will look at me and might think I am ugly, others might think I am pretty, some might think I am fat, others I am skinny, some might think I am smart, others dumb, despite being the same person and presenting the exact same way. to all So why should I put any value onto people's opinion of me. Why should what people who think negatively of me matter more than those who think positively and more than what I think of me?
Once you learn to genuinely not give a fuck (regardless of the situation) you will find a sense of confidence nobody can take away from you.