I don't understand how my mom left myself and my brother when we were young

I'm a new mommy. She will be 4 weeks old on Tuesday and as I'm sitting here patting my baby girls butt after feeding her I just can't help but wonder how my mother, the woman who birthed me, just left. I hate taking a shower because I'm away from my baby for 20 minutes. I have so many people offering to watch her while I go get my nails done or want to have a night to myself with my boyfriend but I just can't think about not being with my little girl for even an hour. How did my mother manage to walk out forever? I'm just glad my daughter will never have to wonder these same things.