Something I hear a lot of during class is "I can't". If it's one thing I've learned over the years of teaching, is you CAN. Maybe not now, maybe not tomorrow; but with enough practice and tenacity you can truly accomplish anything. As an instructor it's my job to help break it down and to find a way of teaching that makes sense to you as the student. I have taught students where they stayed seated in a chair and did the whole class that way, whatever your limitation It's my duty to adjust and find a way to include everyone.
By telling yourself "I can't", you are only dampening your opportunity for growth. You are stopping yourself from accomplishing your goal. Kids hear everything and when you tell yourself over and over again "I can't", you are planting it in the other dancers that it's okay to think and talk to yourself that way. Attitude and energy (good or bad) is contagious.
Here are some things you can tell yourself instead of "I can't".
I give up -> what do I need help with?
This is too hard -> I just need to practice more
I made a mistake -> now I know what to improve and make it better next time
She's better than me -> what is she doing different that can help me?
It's good enough -> I can always improve and make it better
I can't -> yes I can, I just need more time to practice
I suck -> I can't give up, I will get better