What is the way to get rid of the feelings of loneliness and pressure, hatred of life, and the desire to die, that afflict the questioner after her father's death, despite her persistence in prayer?

What you are going through is a great trial, and its reward is immense. Patience with it is a light. The correct way to deal with painful emotions passes through five stages: realizing them, understanding them, and classifying them, then expressing them in a suitable environment, and finally organizing and managing them. Psychotherapy is recommended to deal with these emotions, just as prayer and supplication are advised, along with confiding one's grievances to Allah Almighty.