Hi Kaika,
I think some people think that being grown up is a destination, but let me tell you.... it is a life long journey!!!
I say this because it is not like a great big book entitled,"The Grown-up's Complete Instruction Guide" falls out of the sky and lands in your lap when you turn 18!!
One is
ALWAYS learning throughout one's life!
Also...
EVERYONE has moments when they feel like they are incompetent, or unconfident.
It doesn't matter how educated, successful, or how old a person is... everyone has moments when they feel that way.
So don't be too hard on yourself. Relax and take life one day at time.
Life is just a big journey, that's all... and it can be a very pleasant and enjoyable one if you make the right choices.