My heart is set on obeying

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)

to the very end

Possible meanings are 1) "every one of them" 2) "to the very end of my life" or 3) "to the very end of time"