How can one overcome the intense fear of death and depersonalization disorder (Derealization/Dissociation), and return to a normal life after suffering from sins, anxiety, panic attacks, and disturbing thoughts, especially with the desire for religious commitment and to complete one's studies?

A person finds no happiness except in obedience to Allah and drawing close to Him. Therefore, you must immediately repent of sins by wearing the hijab and quitting smoking. Seek the help of righteous companionship that will assist you in obedience, and adhere to dhikr (remembrance of Allah) and du'a (supplication). Trust in Allah's mercy and forgiveness, for He erases sins through repentance. As for panic and fear, supplicate to Allah to remove them from you, and consult psychiatrists for treatment.