C.C.
Dear D.:
I have breat fed three children, for about two years each, and I am an R.N. Breast feeding is based upon supply and demand. If you start using formula, your baby will suck less and you will then produce less milk. Barring some very specific problem with your body, if you drink lots of water and nurse regularly, your body should produce all the milk that your baby needs. There is no formula that will ever compare to the perfection of breast milk for the human baby.
Unfortunately, many, many women say their bodies cannot produce enough milk, they supplement, their bodies produce less and less, and before you know it, they are using only formula.
Please consider joining the La Leche League, or even calling one of their leaders for free advice, becuase they are a wonderful help to nursing Moms.
Good luck!