Handling 6-year-old's frustrated outbursts: "I'm SO dumb!"

[saffrongirl]

My daughter just turned 6 in December.  She is the oldest child, has always been pretty independent and fairly calm.  She has always seemed to be confident and pretty sure of herself (for example she'll go up to a store clerk to ask question by herself if prompted to).  She will play and work alone as well as with others quite well.  Historically she doesn't really "rock the boat" at school - teachers at her Montessori school comment that she's "sweet" and "quite intelligent."

She has a little sister - almost 18 months old - and two step-siblings (both around her age).    She plays well with them for the most part with the usual rivalries and so on.  

I've noticed in the last six months or so that she's been getting increasingly frustrated and angry when she doesn't do something "correctly".  Montessori teaches that the kids should just learn to sound out a word and for now, it doesn't matter if it's spelled correctly, however, she will ask if it's spelled "right" and I will tell her if it is or isn't (usually saying something like "well most people will spell it like xyz, but I think it's great how you sounded it out!").  She usually gets really angry if she's done it incorrectly.  At one point I even asked her if she wanted to know if it was spelled correctly or if I should just let it go and she said she wanted to be doing it right so I've honored that by telling her when she shows me incorrectly spelled words. 

But lately when she gets upset, she'll shout something like "I'M SO DUMB! EVERYONE IS BETTER THAN ME!" I have voiced varied responses to this, from "oh Maia, you are SO smart and you are only 6 and there's no way you could have known that yet" to "Maia, get over yourself, this is ridiculous and you know you're just saying this for attention."  (Not too proud of that response actually.)

I guess I'm just wondering if a. this is normal and b. what should my response be?  Should I be concerned about her confidence or is she just testing boundaries especially with a new(ish) sister and starting first grade next year? 

Thoughts? 

Thanks!

[HappyMommy2]