I have two that i use all the time.
since i never really paid much attention to school until college i was always really slow i picking up math, science and other crap. It took me 10 times of re-reading the LSAT logic games book instructions to finally understand it. The 9th time it finally made sense and i read it a 10th time to make sure it made sense. sh1t you don't know how pissed and frustrated i was when my friend just picked up the book and started to answer the problems and i had no clue how.
"If someone else can do it, so can I." - this one is for not selling myself short
"Just because someone says they can't do it, doesn't mean i can't." - this one is for all the people that tell me i can't do things or don't believe in my abilities.
What matters is believing in yourself and working hard.