What are the causes of sadness, worry, and fear of the future, what is their cure, do they always accompany a person, and what is the advice regarding this?

We cannot diagnose the condition or determine its cause. We recommend maintaining the prescribed morning and evening adhkar (remembrances of God) and ta'awwudhat (seeking refuge in God), while contemplating their meanings, relying on God, trusting in Him, and firmly believing that all matters are in His hands.

Among the beneficial adhkar and ta'awwudhat are: "I seek refuge in the perfect words of Allah from His wrath and punishment, from the evil of His servants, and from the evil suggestions of the devils and from their presence," and reciting Qul Huwa Allahu Ahad (Surah Al-Ikhlas) and the Mu'awwidhatayn (Surah Al-Falaq and Surah An-Nas) three times in the morning and evening, and saying: "Allah is sufficient for me. There is no god but He. In Him I have placed my trust, and He is the Lord of the Great Throne," seven times in the morning and evening.

Also, among them is saying: "Allah is sufficient for us, and He is the best Disposer of affairs." And saying the supplication for worry and grief: "O Allah, I am Your slave, the son of Your male slave, and the son of Your female slave. My forelock is in Your hand. Your command over me is forever executed, and Your decree concerning me is just. I ask You by every name belonging to You which You have named Yourself with, or taught to anyone of Your creation, or revealed in Your Book, or kept to Yourself in the knowledge of the unseen with You, that You make the Quran the spring of my heart, the light of my chest, the reliever of my sadness, and the remover of my worry."

It is also advisable for you to consult specialists in psychological matters.