Does marriage erase the sins committed during the engagement period, and what are the signs that repentance for these sins has been accepted?

What happened between you two is a transgression of Allah's limits. The engaged couple are strangers to each other, and what you did is not permissible except between spouses. Marriage alone is not enough as an expiation for these sins. What is obligatory is repentance with its conditions, in addition to other expiations for sins such as seeking forgiveness and performing good deeds, along with increasing good acts and having a good opinion of Allah.