What is the ruling on the fear of death, and what should I do when I am going through difficult circumstances?

Let your heart be attached to Allah, trust in His management, and rely upon Him. Your sufficiency from Him will be in proportion to your realization of servitude to Him. Allah Almighty says: "And whoever fears Allah – He will make for him a way out. And will provide for him from where he does not expect. And whoever relies upon Allah – then He is sufficient for him." (Quran 65:2-3). Increase in supererogatory acts of worship and adhere to supplication. Seek happiness in obedience to Allah. Allah Almighty says: "Whoever does righteousness, whether male or female, while he is a believer – We will surely cause him to live a good life." (Quran 16:97). Death is an inescapable destiny, so prepare for it and strive in obedience. The believer remembers death to increase his diligence in obedience. And know that what Allah has decreed will inevitably come to pass, so do not despair. Be at ease with Allah's decreed judgment, and ask Him to keep you alive as long as life is better for you, and to cause you to die if death is better for you.