Is the understanding that a Muslim is someone who allows for their rights to be infringed upon correct, and am I sinful for holding animosity towards one who took a portion of my land, given that I did not relinquish my right?

You are not obligated to forgive the one who wronged you, and you have the right to demand your due without embarrassment, for Allah Almighty says: "And whoever avenges himself after he has been wronged - those have no cause for blame." The oppressor must repent and restore the right; otherwise, what he oppressed will be wrapped around his neck on the Day of Resurrection from seven earths, and his good deeds will be taken from him, or he will be burdened with the wronged person's sins.