Is it obligatory to renew one's Islam and repentance anew after committing a sin that one had sworn not to commit, or is repentance only for the sin itself? And is it possible to renew repentance every night before bed, and how would one do that?

May Allah accept your repentance. Repentance for all sins is accepted, as long as its time has not passed. Among the conditions for an accepted repentance is the firm resolve not to return to the sin, not necessarily the actual non-return to it. If someone pledges to Allah to abandon a sin and then commits it, they must offer expiation. The conditions for the repentance of an apostate have been mentioned. If a person returns to a sin after repenting, it is sufficient for them to repent from the sin they returned to. Renewing repentance is done by seeking forgiveness and supplicating to Allah for its acceptance, and it has no specific time; for the Prophet, peace be upon him, used to repent a hundred times a day, and Allah extends His hand day and night to accept the repentance of the wrongdoer.