Is it possible to switch back to breastmilk after feeding formula? Yes.
Here are 2 articles to help you transition. It would also be good to seek the counsel of a pediatrician and lactation consultant while doing this so they can help you monitor the health of your baby, and help the baby successfully adjust to feeding from a breast, as it is different from a bottle.
If you choose to switch, monitor how your baby is doing and adjust to what keeps her/him healthy. Do your best to eat a nutritious diet, because what you eat gets passed on to the baby. Watch for foods that make your baby irritable, extra gassy, constipated, have diarrhea, cause skin problems, allergic reactions, or any other adverse effects, and avoid them. For the same reasons, be vigilant with products that you put on your hair and skin (i.e. avoid chemicals as much as possible).