We appreciate the sensitive nature of this question.

According to many halachic authorities, relations on onat haveset are prohibited rabbinically. Even according to those who maintain that this halacha is on a Torah level, it is only for a veset kavua and even then not punishable by karet. Halacha also distinguishes between witting and unwitting transgressions.

It’s clear from what you write that you regret what happened and will do your uptmost to prevent it frpom recurring, essential steps of teshuva. Traditional acts of teshuva include tzedaka and tefilla, which we recommend here.

Yehi ratzon she-tizku lemitzvot! Please let us know if we can be of further assistance.