One thing we hear a lot in homeschooling circles is that you are the only teacher your child will ever need. God gave that child to you, so therefore you are the best teacher for them.
In so many ways, the "God gave you that child, so you are equipped to teach them" statement is true, but it falls short in so many other areas, not to mention causing undue grief, guilt, and shame when a homeschooling parent (usually mom) fails to teach her child something on the academic spectrum despite her best efforts.
Each of us is created with unique traits that lead to our development at differing rates, our interest in vastly different subjects, and our desire to develop passions that vary widely from one person to the next. Naturally, then, by the time you become a parent, you are going to have greater knowledge in one subject over another.
The folks who really push the "You should be your child's only teacher" mentality claim that having an academic weakness in an area is really no problem for the parent; they can learn right alongside their child. That sounds really cool and all, but the reality is, we live in reality. It's not the parent's only job to learn calculus a chapter ahead of their student. Most of us also work outside the home, work from home, work at home, have other children in other grades, care for aging parents or kids with special needs, have responsibilities to ministries and organizations, and have to feed everyone come dinnertime.
The other hole in the learn-with-them argument is that our kids aren't perfect. 5 teenagers in to raising our 8 kids, we've seen them all lag academically around 8th grade. No one seems to want to admit this in homeschooling circles, but the truth is that many (most?) teens go through several years of wanting to do anything but schoolwork. That old adage of "herding cats" becomes more like "herding cats in comas." Seriously.
Look, here's the truth: you may not always be the best teacher for your children. You might find it shocking the first time your voracious reader rolls his eyes at the book list set before him, but when that day arrives, look up and remember that you have not failed your kids simply because they seem to have lost their love of learning around 12 or 13 years old.
Now is the time. Pull in an internet course or a tutor or a co-op or a classroom setting like Classical Conversations. Let them be accountable to another teacher besides you. They might still lag and drag their feet and struggle for a few years, but they will learn time management, organization, the unique strengths and weaknesses of different teachers, and a respect for you as they look back on all you have done with them.
What if your child is struggling and she isn't even a teen yet? Relieve yourself of the guilt and find some help. Go about it a different way. Ask God to give you resources. Ignore anyone who tells you that online school or co-ops or charter schools or whatever help you need isn't "real" homeschooling. That's the dumbest thing I've ever heard.
Homeschool in freedom and do what God has called you to do. Love the teachers who come into your kids' lives and recognize that they are a gift. You may not always be the best teacher for your children in every subject, but you are the parent God gave them and there's grace enough to go around!