I'm not that adult that lacks photos of her childhood. I have plenty of photos of my mom and I at birthday parties, family vacations and special gatherings. The problem is, I have very few photos that document our time together; the snuggles, the times she did my hair or comforted me when I got hurt or was having a bad day. I can see these images in my head, but i have no photographic proof that they happened.
If I ask any mom why they think photography is important, they will likely come up with some answers like: to freeze time, capture moments, life goes by so fast and you want to remember, ect. These are all true of course, but another answer is that photos are history. And moms are often the chief historian...which often leaves us out of the moment. Years later when we aren't around anymore, our children will want to see photos that trigger memories and remind them of their childhood....and, selfies or portraits in a field just aren't going to cut it. I don't remember any of the studio family portraits we took over the years as child, but, I do remember how my mom and I used to race outside (she's really fast and would always win) and I would love to have a photo of that moment.
Don't think you are alone in remembering to get in the frame. I have to remind myself regularly not to get wrapped up in how I look or what I wish was different about my circumstances. But, I do it b/c at the end of the day, my kids aren't going to care about all the stuff I'm trying to control. They care about looking back at their childhood and their childhood includes me. I challenge you to do the same. Stop waiting for the perfect time, your perfect weight or the perfect house to get in a photos with your kids.
Hand the camera over to your significant other, an older sibling a stranger or hire a photographer. Just don't forget to document life with you in it. It's needed. It's history.