Mothers worry all the time--it doesn't stop when they are adults . Since you love her so much (you have bonded with her), I am sure you are meeting most of her needs. You are not awful -- it is a 24/7 job and you love her deeply.
Perhaps this study will provide a bit of comfort:
It's nature, not nurture: personality lies in genes, twins study shows - Telegraph
They need our love but they are their own little people. I no longer teach but even under perfect conditions some children are born with learning disabilities, are more stubborn than others, etc--you see it in large families where one child is a handful and the rest are perfect angels. It is good to ask for help from friends and family. It can be helpful to take her to church or join the Y--this could provide you some additional support. Since you truly love her; hopefully, it will all work out. Some of it is out of your hands.
When I taught and at home (I have 2 adult children), the school and my H would always emphasize that it is important to have high expectations for children. However, realistic expections are important too. It is so enjoyable to embrace children as they are though I admit discipline was not my strong suit. Whenever you can, enjoy them while they are young--it does not last long......