Formula due to medical reasons

After two weeks of nursing my little guy and finally getting the hang of things, I had to stop due to medical reasons. I have rheumatoid arthritis that is so severe that if I go without meds, I won't be able to open and close my hands. I was off my meds 8 weeks in order to give breastfeeding a try and my flare up hit me like a ton of bricks a few days ago so my dr said no more breastfeeding and back on my meds So I can take care of my baby safely without pain. I'm devastated and feel like a failure and inadequate. I need some words of encouragement from others doing formula and To know I'm not alone. 

Also on a side note, my little guy downs milk and my dr said give him 2-3 ounces of formula. 3 ounces seems like a lot for two weeks but he seems like he devours it and wants more. How do know your over or under feeding?