How can one deal with the devil's whisperings that lead to questioning the benefit of good deeds and supplicating to Allah for concealment despite not achieving marriage, reaching the point of contemplating suicide, even after repentance, prayer, and drawing closer to Allah?

Good deeds benefit their doer, and Allah is self-sufficient, in no need of any of the worlds. Good deeds are an expression of gratitude to Allah for His blessings, and Allah does not let the reward of those who do good go to waste; rather, He multiplies the reward. The good deeds themselves are by the grace of Allah. One must fear Allah (have taqwa) and be patient with His decree and destiny, for perhaps a delayed marriage is better. Supplication (du'a) will not be lost as long as one does not become impatient and abandon it. Thoughts of suicide are from Satan, and they offer no solution to any problem; instead, they bring Allah's wrath and punishment.