Oh, Lemontart I do feel for you. Your post could have been written by me a few years ago.
I am the eldest daughter of an extremely high-achieving father, and have had the same thoughts as you. I pushed myself beyond my natural limits to achieve, to prove to dad and to everyone, anyone in fact even the man on the street, that I was amazing, talented and utterly capable. I worked my socks off and kept all my doubts and fears (and my god, there were plenty) to myself, not even DH really knew how fragile I felt. I kept thinking any minute, someone will find me out and see me for the fraud I am.
In the end I had a nervous breakdown, followed by severe depression. (I have to add that a) there were other things which triggered it, and b) I am in no way suggesting that this is the way you are going!)
Several years later, I am no longer working and if I'm honest, probably not capable of working now - certainly not in my old career, but probably not at all. In my low moments I feel that I have let everyone down - DH, my children, my family esp my father who had such high hopes. But you know what? It's not true. Just because we haven't achieved what we thought we should achieve, it doesn't mean we are not worthwhile people. We can't all set the world on fire. The fact that you are who you are, you are happily married with two children, you are happy - that is all any of us can ask for. And that is all any of us would want for our children - to be happy in their skin.
It has taken me a long time to accept that I am ordinary, not extraordinary. I'm never going to be a director in some FTSE100 company. I'm never going to speak umpteen different languages. Yes, I read a lot of what would be considered fairly challenging books, but I am equally happy flicking through a Hello magazine. I am happy with looking after my children, happy when I cook a meal that they both like. And my dad, bless him, has been so unbelievably supportive throughout the last few years, has done nothing but show his pride in my survival and joy that I know what happiness is again.
So hold your head up high, you are a good person and I bet if you asked your dad, he would say so too.
Big hugs