In my experience, there are two things new moms feels insecure about: their bodies and how they're doing in their new role as someone's parent. Both are tremendously difficult, although I have to admit I haven't had to deal with the former yet. As an adoptive mom, my body stayed relatively the same (although much more exhausted!), but I definitely had to deal with the latter. Thankfully, there are things every grown-ass man (in my case, my partner) does when a new mom is feeling insecure and, yes, they do make a tremendous difference.
As an adoptive mom, I can say that I'm grateful I didn't have to deal with the physical insecurity that normally comes with a postpartum body. Honestly, the, "Am I a good enough mom?" side of things was difficult enough to handle. Despite my best efforts, it was difficult to avoid thinking that because I didn't grow my baby myself, I wasn't a "real mom." I feared that I was behind or lacking, because I didn't have 40 weeks (more or less) to bond with a growing baby inside my belly, especially when I felt like I wasn't figuring motherhood out fast enough. In other words, if I was experiencing a particularly hard day, I really needed someone to redirect the negative thoughts that were sure to take hold and fill me up with self-doubt.
Enter my loving partner. Thank goodness we had been married long enough for him to spot a meltdown coming a mile away. He's pretty good with a standard pep talk, but he's also good at diversifying his tactics when it comes to helping me feel more secure in my motherhood. He may never (ever) put his shoes away after the millionth time I've asked him to, but he comes through when I really need him.