What is the reason for the sadness and the fluctuating state between hoping for Paradise and fearing Hell, and is it from the soul and Satan, or due to sins?

Sins are the cause of worries and sorrows, and their source is the soul that incites to evil and Satan. For a good life, one must abstain from sins and resort to righteous deeds. Allah Almighty said: "Whoever does righteousness, whether male or female, while he is a believer - We will surely cause him to live a good life." Ibn Al-Qayyim said that a good life is guaranteed for believing doers of good deeds in this world and the Hereafter. The goodness of the soul and the joy of the heart that come from abandoning desires and doubts are the true bliss. The believing, pious people enjoy bliss in this world, in the barzakh (definition) (interim period), and in the Hereafter; they are the people with the best life and the most expansive chests.