Am I the reason for my father's death, and how can I accept this reality and be content with divine decree and destiny?

You must convince yourself that you were not the cause of what happened to your father. You should strive to supplicate for him, asking for mercy and forgiveness, and give abundant charity on his behalf. Cast away these whispers and occupy yourself with what benefits you and your father in this world and the the Hereafter. Beware of objecting to God's decree. Thank God for His provision, and strive to take your medications and consult doctors. Before all that, seek help from God and humble yourself before Him so that He may cure you of this ailment.