I don't know why, but I remember when I was a teenager, I was with a bunch of guys that I did something with, I won't go into details, but it was a group competition. I was two years younger than any of them, the oldest maybe 6 years older than me. I sort of felt that I was the weakest link of the group.
We won.
Afterwards, we went out to celebrate, and one of them told me (talking about the part I played in the victory),
"You know, you really nailed it. You totally aced your part. You did a fantastic job".
So I said, "Oh, not really , I just " blah blah blah.." as I basically said that I didn't do anything special.
And he looked at me, and said, "Listen, when somebody pays you a compliment, don't do that. Listen, then say thank you. It's OK to be good at something and to not be shy about it."
It sort of changed my ideas about being confident, ever since.