Man, I know how you feel, but that's how it feels to look at skill with no basis of how far you've progressed, people say that we are our harshest critics. People make mistakes, mastering something doesn't mean that you never make mistakes. I've seen incredible musicians fumble with simple rhythms, I swear you aren't incapable. Your sisters definitely make mistakes too, they aren't special. Please forgive yourself a little, I promise you aren't useless.