Don't swim against the current. Just know that you are doing what you have to do, and you're doing it as well as you are able. You're not Superwoman and no one expects you to be.
Single working moms are teaching their kids a solid work ethic; you're teaching them that we don't just have things handed to us, and have to work for the things we want. If they are properly cared for while you're out earning the money you need to get by, why should you feel guilty? James Baldwin is famously quoted as saying, "Children have never been very good at listening to their elders, but they have never failed to imitate them."Make the most of the time you do get to spend with them, and know that while you're working, you're taking care of their physical needs AND teaching them an important life lesson.
And finally... "Working mothers are guinea pigs in a scientific experiment to show that sleep is not necessary to human life." - Anonymous