The word for "unable to change"

Question

Is there any word in English for a person who is unable to change himself for someone else regardless of he/she wants to change or not? He just can't change.

I'm writing a play and I need that word for a specific scene if there's any. The scene is, boy B and girl G are in love. Both love each other. But the problem is B can't change his bad habits like smoking and gambling etc. But sure he wants to change. But he's just too addicted and he can't. So G will get the dialog "Look I still love you, but you are too [something something]. You can't change yourself even for your love". I need something to fill in brackets. I thought of stubborn.

Answer 1

• Inflexible - unwilling to change or compromise.
  By conscious choice someone is unwilling to change.
  "We must have Christmas dinner at his house every year. He is completely inflexible to having anywhere else."

• Unmovable - incapable of being moved.
  Can apply to inanimate objects as well as people as simply a state of being.
  "The tree is unmovable therefore we must build the path around it."

• Independent - not influenced or controlled by others.
  Mostly a positive characteristic of someone who is self-sufficient.
  "She's very independent. She finishes her homework and chores without ever being asked to."

• Unbending - rigid; refusing to bend or compromise.
  Can apply to inanimate objects as well as people as simply a state of being.
  "The seller is unbending--she refuses to come down on the price."

• Obstinate - firmly or stubbornly adhering to a purpose.
  A more negative characteristic in that there may not be a healthy or logical reason for holding their ground so firmly.
  "The toddler was obstinate about not eating broccoli."

• Unyielding - hard; not apt to give under pressure.
  Someone strong in their convictions or position, either wisely or unwisely.
  "The wrestler's hold was unyielding. His opponent would have to tap out."

• Uncompromising - absolute; without exception.
  Knows what they want and will take nothing else.
  "The chef is uncompromising when it comes to the quality of her ingredients."

Answer 2

I would say incorrigible.

Cambridge Dictionary gives the following for its definition:

(of a person and their behavior) impossible to improve or correct: an incorrigible liar

It also gives the following explanation:

An incorrigible person or incorrigible behaviour is bad and impossible to change or improve.

Answer 3

There isn't a precise word for that in the English language. Most would probably use an idiom like "a leopard can't change its spots" to express that notion. I would suggest rephrasing the line.