Formula

Let me start by saying I breastfed my other two kids for a year and had no problems. My little one is almost 3 weeks old and I've had nothing but problems this time around. My nipples cracked and one side healed, but the other side is still wide open and right in the middle of the nipple so it splits open every time she feeds. So painful. Then I got mastitis and just finished the 10 days of antibiotics. Then, to add insult to injury, at her two week check up, she had not gained any weight since her 4 day check up. I have to go back tomorrow to make sure she's gaining weight. I am now seriously contemplating just giving formula. My question is, since I've never given my kids formula, how much do you feed them? Ounces? How often? With breastfeeding, I just feed on demand. But not as easy to do with formula, I'm assuming. Any help or suggestions are welcome.