What is the ruling on repeatedly committing a sin and repenting from it, and does this expel the doer from the mercy of Allah, especially given the covenant with Allah and the angels not to return to sins?

What you speak to yourself of the whisperings of Satan, to make you despair of Allah's mercy, and your covenant with Allah and your breaking it, is a sin that necessitates repentance. Therefore, adhere to repentance from every sin and strive against yourself to abandon disobedience, for whoever strives against himself, Allah will guide him.