How can one atone for repeatedly succumbing to weakness in front of one's fiancé and coming close to committing a sin? And would that be through fasting or charity?

The two individuals engaged to be married are considered strangers to each other before the marriage contract (Nikah) is solemnized, and therefore, it is not permissible for them to be in seclusion together. Whoever has fallen into such a situation must perform sincere repentance (Tawbah Nasuh) and seek forgiveness. The conditions for repentance are: abstaining from the sin, regretting it, resolving not to return to it, and that the repentance be made at its proper time. It is advised to increase good deeds, not to return to the sin, and to expedite the marriage if possible. Otherwise, it is necessary to maintain distance from the suitor until the marriage contract is finalized.