As a new mom, it's pretty normal to question your capabilities on a frequent (if not daily) basis. Just like your first job or your first year of school or your first relationship or your first anything; you're not necessarily sure you know what you're doing. You have a general idea, sure, but it's still foreign. Thankfully, there are a number of little things that can give a new mom confidence, so those self-doubts are (hopefully) few and far between.
When I became a mom, I felt like I had no idea what I was doing. I am the first of my group of friends to have a child and the first in my family to have a baby since my generation was born. In this world of ready-to-fire criticism any time you do or say something even slightly controversial (whether that be breastfeeding in public or private, formula feeding instead of breastfeeding, birthing with an epidural or birthing at home with a midwife), everyone is ready and willing (and sadly, able) to tear you down and tell you how you should be parenting, and why. Coupled with the social expectations our culture places on new mothers and, well, it can be difficult to feel completely confident in your abilities, even when you know you're doing it right.
While I think it's important to voice those doubts and acknowledge that, no, motherhood isn't necessarily innate and we don't always know what we're doing all the time, it's also worth noting the times that can boost your self-confidence and make you feel like the capable parent you are. We need to take it easy on ourselves, as new mothers and as women, so know that even when you feel your lowest, you're doing a great job, mom.