i've searched "breast milk nutrition" in Google for you and found many professional articles supporting breastfeeding for over 1 year older. You may also have a look at them and they certainly will confirm what you are doing is right.
most of the articles state that breast milk is perfect for babies and there are still many nutrients that are unknown which can't be imitated by formula milk.
formula seemed to have more nutrition as they can't be fully absorbed and thus taken into account the "nutrition to be lost".
I am sure that the breast milk is better but the only problem is -- too tired. :-(