What caused the change in the committed young man's condition after his engagement and desire to marry, and does his marriage play a significant role in that?

You have fallen into a tribulation after your engagement, which led to a change in your state from commitment to laxity in religious matters. This is evidence that you have been afflicted by tribulation, as Hudhaifah ibn al-Yaman (may Allah be pleased with him) said: "Whoever wishes to know whether tribulation has afflicted him or not, let him look: if he sees something he once considered lawful as unlawful, or something he once considered unlawful as lawful, then let him know that it has afflicted him."

To escape this tribulation, you must do the following: 1. Supplication: Humbly implore Allah to turn away tribulations from you and to firm your heart upon faith. 2. Islamic Knowledge: Learning your religion protects you from error and reminds you of the brevity of this world and the inevitability of death. 3. Acts of Obedience and Righteous Deeds: Return to your previous state of worship and increase it, such as reciting the Quran, [performing] remembrances (dhikr), and [engaging in] righteous deeds. 4. Righteous Companionship: Adhere to those who help you obey Allah and remind you of Him, and avoid bad company. 5. Fear of an Evil End: Remember that a person dies in the state they lived in, so be keen to repent before it is too late.

True happiness lies in faith and closeness to Allah. A righteous wife is an aid for a man in his religion. The Prophet (peace and blessings of Allah be upon him) said: "This world is a provision, and the best provision of this world is a righteous woman."

If your fiancée is the cause of your relapse, try to reform yourself first, then try to bring her closer to commitment. If she responds, then all praise is due to Allah. If she does not respond, then separating from her is better for you and for the soundness of your religion, because a believing wife helps her husband in his faith.