T.M.
I have always worked full-time as a mother and felt guilty about not spending enough quality time, or time at all, with my son (now eight). Clearly having a baby (first, second, or third) is a very personal decision so it goes without saying that I'm just giving you my perspective.
What I would say about having another child is, giving your child a sibling is not an important enough reason to bring another child into the world. So many people say that and to me, it's just not valid. A child should be brought into the world when you (and your husband, since you have one) feel a gaping void in your family. It doesn't seem like that is the case. You are overwhelmed as it is with what you have (balancing work, house, family time), and you fully enjoy the one child you have without missing another one. All you feel with respect to another baby is guilt over your child not having a sibling. Guilt is not wanting another child. Your child gets companionship and the lessons she needs about getting along with other children from her daycare situation. You don't mention if she has cousins but if she does, perhaps having her get very close to them can also serve as siblings. You enjoy her and you do everything you do with her with joy. THAT is what parenthood is about; not being harried and stressed because you have a new baby and can't balance your toddler and infant.
As the children get older I'm told it does get easier so maybe that is when you will feel the void and want to have another baby. But until that moment comes (if it does) I would say your one child is the one you should enjoy and balance life with.