How can the condition of "no return" be achieved in repentance from sins committed during the engagement period after marriage, and does the inability to return to the same situation affect the validity of the repentance, causing regret and distress in married life?

One of the conditions for repentance is the firm resolve not to return to the sin. This is an act of the heart, meaning an inclination towards something and a determination to do it. It is sufficient for the questioner to have in their heart a choice and preference to abandon the sin if time were to rewind or if it were to be presented to them in the future. So, repentance is the choice to abandon a past sin, whether literally or figuratively, for the sake of Allah. This includes even those who cannot return to the sin. Be steadfast and rejoice in Allah's vast mercy, for Allah forgives all sins. We advise you to frequently seek forgiveness and strive in acts of obedience, for good deeds erase bad deeds.