The writer always cherishing and obeying God's decrees is spoken of as if the decrees are land or a possession that the writer will inherit. AT: "Your laws will belong to me forever" or "Your covenant decrees are like a heritage I will keep forever" (See: [[rc://en/ta/man/translate/figs-metaphor]])
Here "heart" represents the writer's will. The phrase "heart is set" is an idiom. It is a way of saying the writer is determined. AT: "I am determined to obey" (See: [[rc://en/ta/man/translate/figs-metonymy]] and [[rc://en/ta/man/translate/figs-idiom]])