What is the ruling on a forbidden sexual relationship with a man that has lasted four years, despite their religious commitment?

Adultery is one of the most heinous sins and greatest major transgressions that incur Allah's wrath, and its perpetrator is punished in this world and the Hereafter. Islamic law has prohibited everything that leads to adultery, such as a man and an unrelated woman being alone together (khalwah), and their conversing unless there is a need and with modesty. It has also commanded lowering the gaze and observing hijab, due to the fact that freely looking at forbidden things is one of the causes of falling into indecency. Therefore, it is obligatory to cut off the forbidden relationship and immediately repent to Allah Almighty by abandoning the sin, regretting having committed it, resolving never to return to it, avoiding the causes of disobedience, and frequently engaging in remembrance of Allah and seeking His forgiveness.