We can all understand your worry, mama. It's very hard to be a baby's sole food supply. It requires you to trust your body to be able to meet your baby's needs- and that's a difficult leap of faith for a lot of people.
If you do decide you would like to bring the proportion of breastmilk in your baby's diet up, or transition back to exclusive breastfeeding, this is a really helpful link: http://kellymom.com/bf/got-milk/basi...rease-formula/. Basically, you'd be cutting out a little bit of formula each day and increasing the amount of nursing you're doing. The increased stimulation will lead to increased supply.