new baby and failing to feed

I had my baby on Wednesday, at first she was doing well latching and feeding on me. Since yesterday it seems she has given up. My milk has not come in yet and I know she needs to eat so I’ve given formula (but only after trying very hard to use the breast first). The problem with the formula is that she spits up a ton. Please give me some advice, I want to be able to provide for her 😭