What is the ruling on one who invokes upon himself expulsion from Allah's mercy if he returns to a sin, and then he returns to it?

A Muslim must avoid sins. This is even more imperative if one has sworn an oath to avoid them. You have erred by committing the sin, by not fulfilling your oath, and by supplicating against yourself, which is forbidden. Therefore, sincere repentance to Allah is obligatory, and one must not despair of His vast mercy, for Allah the Almighty said: ﴿Say, "O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah . Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful."﴾ You should supplicate frequently for steadfastness, occupy yourself with beneficial knowledge and righteous deeds, and keep company with the righteous. Additionally, the expiation for breaking an oath (kaffarat al-yameen) is obligatory. If oaths are repeated concerning the same matter, only one expiation is required, unless the subsequent oaths were made after the initial one was broken.