As long as the individual is able to refrain from what could cause his death, he should protect himself. If he did not and performed what led to his death, he will be held accountable for making such decisions.
One’s pre-destined death by Almighty Allah does not make him unaccountable for such negligence towards himself, as long as he is able to prevent himself from the causes of death.

There is no doubt that humans act out of their choice and their will. Thus, the good-doer is praised and the evil-doer is dispraised. If humans were like machines such that their acts are under compulsion, then praising those who do good and dispraising those who commit bad deeds would be meaningless. However, all of this is within the boundaries of Almighty God’s power. He gave the human beings the freedom of choice and will hold them accountable for it, and He can take this ability away from anyone whenever He wishes, and there is no power or strength but from God Almighty.

The Imams of the Ahlulbait (peace be upon them) explained this belief in their famous narration: “There is no compulsion [from Allah on the humans], nor there is complete delegation (from Him for them without them being under His power), but a matter in between.”

The belief of "Qadha" and "Qadar" (fatalism and destiny) in the teachings of the Ahlulbait (peace be upon them) is "al-Amr baynal-Amrain" - a path between absolute fatalism and absolute freedom. This means that the human being is capable of the actions, has a choice in doing them and is responsible for their consequences; but at the same time, he is under the power of Almighty Allah, since the abilities of the human being is not independent from His power. The human capability is limited by the creation of Almighty Allah and His continuous maintaining of the life of the human being.

Some traditions indicate that marriage is one of the reasons for the increase of sustenance, but this does not mean that it is a complete cause for it. There might be obstacles that prevent it or conditions of sustenance that are not met. On the other hand, Almighty God might limit one’s sustenance for various reasons, such as to test one's faith or increase one’s rewards if one is patient, or other reasons that we are ignorant of.
So, we should always turn to Him in all our affairs and ask Him to provide us with what is lawful and what would be good for us. Asking forgiveness is one of the means to increase sustenance and thanking the Almighty for His bounties is one of the causes for the continuance of blessings, as the narrations indicate, so we must do what we can from that.
In addition to this, one can recite supplications that help in sustenance, which are mentioned in the books of supplications. If the supplication is answered, we thank God for that. If it isn't, we have to be patient, as it is perhaps will be good for us.

Allah Almighty created this life for testing and trialing us; He did not create it to be a place of happiness. Rather, eternal happiness is in the Hereafter after passing the tests of this life. Almighty Allah might create a person without the sense of sight or hearing, or create a person that suffers from chronic disease, or create a healthy person who lives in poverty, or one that suffers an accident that results in losing a limb or a sense, or any other trials and tests that people undertake.
One should respond to that with gratitude for the other blessings that God has bestowed upon him and with patience over his suffering, so he that he becomes deserving of the rewards of gratitude and patience. This itself will be the means to alleviate his suffering and pain, as long as he believes that all of this is in the hands of Almighty Allah, the Merciful.