It is not unusual to have doubts about one's self as we all suffer from the frailties of being human.
I too had standards that were expected to be met. All too many times I did not come up to those standards and the messages I heard over and over created the self doubt. "You have so much potential." "Why can't you be more like your brother." "I expected more from you." And so on. I can only assume you know the routine oh so well.
Eventually I decided that it was my life and I was going to do what I felt was right for me. For example. My older brother was the star of the basketball team. I was tall and fairly coordinated, therefore I should play b'ball too. I did for 2 years and then finally told my father that I was not going to play, I was going to be in the school play. It was my first break through. My first step in becoming my own person and living my life, not theirs.
As I grew older I continued to hone my skills at this. I did make some mistakes, sometimes with less than desirable results. Another example is the trouble I ran into with the legal system as a result of my wanting to be a pot smoking hippie. Sometimes we can't just do what we want. But living my life the way I wanted to as opposed to fulfilling someone else's dreams and hopes. Often it is the other persons desire to have their goals filled by their son, daughter, spouse, etc. My first wife thought I would be perfect if I did this or that differently. If only I would change, I fulfill her desires to become the perfect husband. The more I tried to please her, the more I lost my self. It was the perfect formula for an unhappy marriage. (My present marriage is wonderful. We love each other for who we are and though we are not perfect we accept that fact and indeed, we relish it.)
The fact that I have a blood relationship to my family does not mean I have to like them. I am a product of their sexual activity, early input of nutrition, ideas and values but I also grew and had my own thoughts and desires. I keep what I learned from them that I deem good and useful. (They did have some good things going for themselves. There is always input, whether from family, friends, strangers, this forum. It is my job to sort through it all and decide what is useful.) Though I have written off some of my blood family I am now a fairly well adjusted human being. I am not perfect, I am me. You are not perfect, you are you and that is wonderful.
I hope that by sharing my experience you can make some headway with your dilemma. We only have one head and often there is not room to let others take up space in it. Hang in there.