I breastfed my daughter for 18 months and I have to say, I don't think you need to worry at all. Of course you have to alternate breasts when nursing only one child, but it's not a catastrophe if you forget which you "used" last. The body is an absolutely amazing thing and will make as much or as little milk is necessary to provide the appropriate amount of nutrition for your child. The only time I had any pain was in the very beginning with being engorged while my body was becomming acclimated with the amount of milk needed. After about 3 days this went away and it was fine for the remainder of my nursing. Good Luck!!