inveterate
We were surprised at her inveteracy against poor Sarah.
The moral inveteracy of the main character proves to be an ethical and interpretive problem for the novel.
She soon made the inveteracy of her feelings abundantly clear.
By the inveteracy of their hostility, they had provoked his resentment.

SMART Vocabulary: related words and phrases

Strength of will and determination
aggressiveness
bitter
bloody-mindedness
bull
bull something through
character-building
fibre
insusceptible
intransigence
mettle
militancy
obstinacy
resolution
sharp
sharp elbowsidiom
sharpen
sharpen (something) up
sharpen your elbowsidiom
single-mindedness
stamina

You can also find related words, phrases, and synonyms in the topics:

Strong-willed
Keeping and staying the same