Good mothering??

It is school holidays and these holidays have left me wondering about good mothers. Do mothers EVER feel like they are 'good Mothers'? Even those mothers who look like they have it all together. I know we all love our children and only want the best for them, but it's more than that that makes a good parent right? 
I used to look at the child throwing a tantrum at the shops and wonder what their parents were doing wrong. Then...I became a parent myself!!
There is clearly no secret formula and what works for one child has zero effect on another. I do wonder at the end of some days whether or not I am a good mother though-cos some days I certainly dont feel it!
Locking myself in the bathroom for a few minutes peace DOES NOT make me feel like a good mother.
A bag of Cheezels and 2 minute noodles for lunch DOES NOT make me feel like a good mother. 
Putting my boys to bed and realizing I forgot to brush their teeth DOES NOT make me feel like a good mother.
But then I began to wonder if even good mothers feel this same way. Don't we all worry about the fine line between giving our children nice things and spoiling them? Are our discipline methods too tough or too relaxed? Should I bother wiping those handprints off the sliding door knowing they will probably end up back there tomorrow? If I shove toys under the bed for now rather than putting them away, will it really matter? Is fairybread considered a sandwich or more of a dessert?
So what is it that makes a good mother?
Maybe it's simply the fact that we worry about being good mothers that is the sign of a good mother after all.