Why the guilt?

Why is it that we, as mothers, are so stinking hard on ourselves? So often at the end of the day, right before my husband gets home, I feel like a failure? My house is a mess; I didn't spend enough quality time with each child; S didn't eat a single vegetable; I gave M a bottle because I just couldn't stand to hear him cry anymore; and the list goes on.
Now, I saw/heard of a study... Now, I should preface this. I don't remember where this came from. It was probably a segment on the Today show, which I'm addicted to. I hate it when people start quoting things that they don't have references to (especially in Sunday School), but I'm going to do it anyway. So, back to the beginning. I saw something on the Today show probably that compared the amount of time mothers spend with their children. Just for illustration (these numbers are NOT right), let's say in the 1950s women spent 10 hours a week with their kids, and today women spend 14 hours with their kids. Yet, women today feel much guiltier today than 60 years ago. WHY is that?!
Typical day for us: I get woken up at 5 to feed a baby. Go back to bed. S gets out of bed before I go back to sleep. Put her back to bed. Go back to bed. Before I get back to sleep it's 6am and S's up again. I get her up and ready for the day. M wakes up. I get him ready for the day. It's now after seven. I help N get off, put M down for a nap. I know get to eat finally. It's 7:30 or 8:00 and I've been awake and starving since 5. I try and watch the news while eating breakfast but S is all over me, putting her fingers into my cereal and getting whiny and mischievous. I feel guilty and frustrated and turn off the news. Why?
And the day continues in a similar fashion. Much of my frustration stems from the fact that my darling little girl won't be separated from my side for a second. She's constantly following me, even if it means leaving her snack or tea party. It doesn't matter that I tell her I'm just going to put M's pacifier back in or go brush my teeth and I'll be right back. Nor does it matter that I'm already going back downstairs by the time she reaches the top. So, for instance, today while I was cleaning my room (which needed to be done very badly) she insisted on being there instead of playing by herself. This resulted in nothing being done satisfactorily. I wasn't working effectively because I was constantly pulling S out of something that I had already cleaned up and telling her 'no.' She was getting frustrated because I wouldn't play or let her play with anything. Never mind that I would have time to play with her if she would just go away and play by herself while I finished the job. And in all of this I feel guilty.
So why do we do it? Yes, we're not perfect, but no one expects us to be. So how on earth are we supposed to find a balance between taking care of and playing with our kids and working and cleaning the house? Oh, and maybe do something for ourselves every now and then - preferably guilt free.