For those who were pretty insecure about yourself, how did you become confident?

So I'm pretty much insecure about myself. I don't like how I look, I hate that I'm having no clue where to build that confidence and seeing other girls my age doing so many fun things, I just feel so insecure that I'm still a child and it's hard. How do I be confident and show off this to others without manifesting my inner insecurities?

I have been going through so much FOMO (please don't judge me, I know) that I did something like going to the pub for the first time, tried a few drinks (although I'm super new to it) and even visiting a few clubs with friends. But everytime I go, I feel that I'm not smart enough and keep on comparing myself with the other girls (I'm jealous of them really lol)

How do I stop these thoughts and be confident about myself? I don't think I have many qualities which is considered attractive. Please help if you can and share your experiences.