Is it possible to repent and be forgiven for a sin after saying, "May Allah curse me if I am a liar" in an emotional context where there was no true love, especially now feeling remorse?

It is not permissible for a Muslim to curse himself or others, because cursing is expulsion from the mercy of Allah, and his supplication might be answered, leading to the curse befalling him. This is based on the Prophet's (peace and blessings be upon him) saying: "Do not supplicate against yourselves, nor supplicate against your children, nor supplicate against your wealth. Do not choose an hour in which Allah is asked for a bounty and He answers you." It is obligatory to repent from cursing oneself, lying, and forbidden relationships.