Even here though, the CONTENT did not really affect your Positive Self Image belief, but the EMOTIONAL TONE certainly did.
Supposing your mathematics mistress took you to one side and said to you very kindly: "Although you have a lively and inquiring mind, and despite the fact that you have tried really hard this year, I feel that maths is probably not the subject for you. However, your many other abilities outweigh this and I have suggested to the headmistress that you be allowed to join the fourth year Latin class."
The CONTENT of this message is a bit of a downer. She is effectively saying that you are useless at maths, AND THIS WILL BE NOTED WELL BY THE I-CAN. However, she has not really affected your feeling of self-worth because her EMOTIONAL TONE suggested that she liked you, and that you were a really worthwhile person.
Contrast this with the same treatment your friend receives at the hand of the dreaded Mr Masters: "Pay attention Jones, you disgusting, snivelling little creep! Just because you're top of the class and get every answer right, doesn't give a smart-ass like you permission to dream your worthless life away staring through the window!"
A little extreme perhaps, but the CONTENT was effectively telling Jones that he was really good at maths! (AND THIS WILL BE NOTED BY HIS I-CAN), however, how do you think the message affected his Positive Self Image? BADLY! His Positive Self Image belief only heard the EMOTIONAL TONE of the exchange, which effectively told Jones that he was not a worthwhile person, and that the master despised and loathed him.
His Positive Self Image belief will be greatly reduced if exposed to too much of this type of treatment - particularly if it is being re-enforced in his home life.