I just feel like a bad mom

My LO is almost 6 months. Some days I feel like I can do anything and do it all. I dream about giving her a brother or a sister. But then I have days like today where she's not napping and just screaming no matter what I do. I get so frustrated. I never yell at her but I mumble things like "just go to sleep" or "please stop crying." Then I feel awful and like I can't even handle her rough patch how can I handle another baby?  I haven't felt this way in a while and feel so bad today.