what does "The iron has entered into my soul"here mean?
"I'm awfully sorry I made fun of your hair, Anne," he whispered
contritely. "Honest I am. Don't be mad for keeps, now."

Anne swept by disdainfully, without look or sign of hearing. "Oh
how could you, Anne?" breathed Diana as they went down the road half
reproachfully, half admiringly. Diana felt that SHE could never have
resisted Gilbert's plea.

"I shall never forgive Gilbert Blythe," said Anne firmly. "And Mr.
Phillips spelled my name without an e, too. The iron has entered into my
soul, Diana."

Question

what does "The iron has entered into my soul"here mean?
"I'm awfully sorry I made fun of your hair, Anne," he whispered
contritely. "Honest I am. Don't be mad for keeps, now."

Anne swept by disdainfully, without look or sign of hearing. "Oh
how could you, Anne?" breathed Diana as they went down the road half
reproachfully, half admiringly. Diana felt that SHE could never have
resisted Gilbert's plea.

"I shall never forgive Gilbert Blythe," said Anne firmly. "And Mr.
Phillips spelled my name without an e, too. The iron has entered into my
soul, Diana."

Answer

I'm an Australian, and I have heard a similar expression.  I straight away thought of a branding iron, that has a symbol on it.  The iron is heated up and forced onto the skin, marking a burn mark in the shape of the symbol.  Used for farm animals sometimes. It hurts no doubt, so I believe the iron has entered right into the soul is talking about a deep wound or hurt, not just a skin deep insult.

Answer

It means that she has been hurt.

Answer

thank you so much

Answer

I had never heard this expression before, and I am a native UK English speaker.

A quick bit of Googling gave me this:

https://idioms.thefreedictionary.com/the+iron+entered+into+someone%27s+soul

Today I learned something new!!