Final answer:
The quote that best supports the idea of feeling regret for acting against one's character is 'I failed you, dear lady… I failed myself.'
Step-by-step explanation:
The quote from the text that best supports the idea that a person can feel regretful when they act against their character, even temporarily, is:
"I failed you, dear lady… I failed myself."
This quote shows that the person feels regret for not living up to their own standards and letting someone down. It suggests that they acted in a way that goes against their character and they are feeling remorseful about it.