How can a committed young woman overcome the state of breakdown she is experiencing after her fiancé left her, especially with her repeated attempts at fasting, prayer, and supplication, and is there any advice to end her thoughts of returning to him?

One should feel that everything is by the decree of Allah, and that whatever He decrees for the believer is good for him, so there is no sorrow over what has passed. You must be patient, for it is the key to many doors of goodness. Increase your remembrance of Allah, for it brings tranquility to the heart, and continue to supplicate for an easy marriage to a righteous man. There is no harm in seeking assistance from relatives and female friends to search for a righteous husband. For more details, you can refer to fatwas regarding the ruling on love before marriage and how to treat ardent love.