How can one get rid of satanic whispers, intense fear, and pessimism, and achieve psychological comfort, reassurance, and happiness?

In summary, one must turn away from waswas (definition) (whispers of Satan) and not dwell on them. This is the greatest cure for them, after remembering Allah and seeking His help, along with supplicating to Him to thwart the devil's plot. Joy and happiness are found in turning to Allah and fulfilling His commands. Allah the Almighty has placed contentment and happiness in His obedience. Allah, the Most High, says: "Whoever does righteousness, whether male or female, while he is a believer - We will surely cause him to live a good life." (An-Nahl: 97). And He says: "Then whoever follows My guidance will neither go astray nor suffer." (Taha: 123). And Allah has made anxiety, sorrow, and distress a result of neglecting obedience and committing disobedience. Allah, the Most High, says: "But whoever turns away from My remembrance - indeed, he will have a restricted life, and We will gather him on the Day of Resurrection blind." (Taha: 124). And whatever anxiety, sorrow, and depression afflicts a person is due to past sins, and it is an expiation for him by Allah's grace. Therefore, you must frequently remember Allah, perform night prayers, and seek forgiveness during the pre-dawn hours to increase tranquility and serenity, and to cut off waswas and not dwell on them.