What is the ruling of Sharia on a wife who doubts her husband after 18 years of marriage, even though he swore to her on the Holy Quran that he has no relationship with another woman?

It is the wife's duty to think well of her husband and of Muslims in general, because suspicion of them is forbidden and considered one of the major sins. Therefore, she must repent of this sin, offer advice to her husband, accept his excuse and believe him, and distance herself from whispers and doubts that open the door to conflict, and cooperate with him in righteousness and piety.