What should I do about my parents' rejection of my marriage to a righteous girl I love, as they threaten to break up the family if I do not comply with their request?

If the girl is religious, of good character, and a memorizer of the Book of Allah, then she is suitable for marriage, due to the saying of the Prophet, peace and blessings be upon him: "Choose the religious woman, and may your hands be dusted (i.e., you will prosper)."

It is recommended to supplicate frequently and seek assistance from virtuous people to gain the parents' approval.

If the parents refuse without a valid reason, then obedience to them in abandoning marriage to her is not obligatory. However, if you prefer to seek their pleasure and look for someone else, that might be better, as Allah Almighty says: "But perhaps you hate a thing and it is good for you; and perhaps you love a thing and it is bad for you. And Allah knows, while you know not."