Here's a few ideas:
Encourage the setting of small manageable goals and then celebrate victories - "How about when you get half way through we have a trampoline break?"
Note success - "today you stuck with it for five minutes without complaint - way to go!" "I remember last week you thought you'd never get them and you got every problem on the page."
Give specific strategies for coping with frustration - punch a pillow, throw a ball, stick a pin in your teacher voodoo doll (okay maybe that's not totally appropriate).
Take on a new challenge yourself (learn to crochet, play the fiddle, whatever). It will make you more empathetic and it is great way to model appropriate behavior.
Teach your child about negative self talk. When she's saying it is too hard is that encouraging herself? What other messages can she focus on? This book might help:
http://www.amazon.com/What-When-You-Grumble-Much/dp/1591474507 Not exactly on the same topic but I found this book helpful in understanding how to encourage positive thinking.
http://www.amazon.com/Raising-Optim...mp;ie=UTF8&qid=1283297591&sr=1-1
