Breast to Formula

Hey ladies! So my baby girl will be 3 weeks old on Thursday. I've been only feeding her breastmilk since she's been born. At night, I feed her pumped milk from a bottle so she gets fuller and sleeps longer. She's up to about 3 oz at night from a bottle, but lately I feel that no matter how much I feed her from me or the 3 oz, it's just not enough. I feel like she's constantly hungry. This plus other stress that I've had about breastfeeding has made me seriously consider switching her to formula. I just have no idea how to do that. Any suggestions?