The glorious Qur'an Chapter 2 Verse 286 is your answer.
Chapter 2: Verse 286
Allah does not charge a soul except [with that within] its capacity. It will have [the consequence of] what [good] it has gained, and it will bear [the consequence of] what [evil] it has earned. "Our Lord, do not impose blame upon us if we have forgotten or erred. Our Lord, and lay not upon us a burden like that which You laid upon those before us. Our Lord, and burden us not with that which we have no ability to bear. And pardon us; and forgive us; and have mercy upon us. You are our protector, so give us victory over the disbelieving people
No soul is burdened with more than what it can bear. So relax. This life is a test. For a believer both good and bad is good. When good happens to him he thanks Allah and when bad reaches him he does sabr expecting reward from Allah. So such weak people are at times a test for those around them. Whether we have patience with them and deal with them with compassion or not is our test. What you think as complete failure, is a test from Allah.
Even after all the good deeds we do, ultimately it is the mercy of Allah that makes us enter Paradise. So a believer spends his life balancing between fear and hope. Fear that he may not have done enough to earn entry to paradise..and hope that Allah will forgive any shortcomings and have mercy on him and grace him with a place in paradise.
In a Sahih Hadith, the Prophet Muhammad, Peace and Blessings of Allah be upon him, his family and companions, said:“Amazing is the affair of the believer. This is because there is good for him in every matter and this is not the case with anyone other than the believer.
When he is in a state of happiness, he is thankful to Allah, and within that there is good for him, and if he is harmed, then he keeps to being patient and thus there is good for him in that.”