Does the believer regret the misfortunes and what he missed out on in this world, and is the Muslim's regret limited to neglecting obedience and committing sins?

Sorrow over what has been missed in worldly matters is often blameworthy and harmful, whereas sorrow over what has been missed in matters of the Hereafter is beneficial and praiseworthy. Thus, a wise person grieves over their shortcomings in acts of obedience and their sins.

Belief in divine decree and destiny removes or alleviates regret over what has been missed in this world, preventing the servant from falling into forbidden discontent and impatience. Patience and contentment do not contradict feelings of pain and sadness.

The verse, "So that you do not grieve over what has escaped you, nor exult in what He has given you," forbids a type of sorrow that eliminates patience and submission to Allah’s command, and a type of joy that distracts from gratitude. It does not forbid the mere existence of sorrow or joy. Permissible sorrow is that which does not exceed the bounds of moderation.