Is despairing of Allah's mercy in worldly matters or of repentance, along with the absence of hope, considered disbelief (kufr)? Is what is meant by "al-qunūt" (despair) only despair of repentance? Is there a difference of opinion on the matter when hope or fear are completely absent? And is it considered disbelief by consensus?

Despairing of Allah Almighty's mercy is considered a major sin if it does not involve denying its vastness. It is blameworthy in matters of both this world and the hereafter. To be freed from this despair, one should reflect on Allah's mercy and blessings. What you perceive as despair might merely be whisperings (waswas), so you must repel them and not succumb to them.