How can one deal with and be patient regarding a mother's terminal illness, given that the questioner is aware of the seriousness of the disease while the mother is not, and the questioner is facing feelings of despair?

A Muslim must be certain that whatever befalls them is by the decree of Allah, and that all of Allah's decrees are good for a Muslim, for calamities expiate sins and raise one's status. There is no meaning in despair and discontent, because that nullifies the reward. The wise person is one who is patient and seeks recompense from Allah. People's reactions to calamities fall into four categories: discontent, patience, contentment, and gratitude. As for how to deal with your mother in her illness, you must be kind and gentle with her, remind her of the reward for patience, and boost her morale by assuring her of Allah's power to heal, while cautioning her against discontent.