I am sick and I feel sad and cry a lot. I know that we all must die and we should praise Allah, pray, make dhikr, read Qur'an and not be sad. But I cannot help feeling scared and sad; what should I do?

Is it haram to be sad and scared of death?

It is not haram to be sad and fearful about death. It is a good thing, which should make you prepare yourself well for your eventual end. Make the best of the time and good health which Allah has blessed you with now, before it is lost and you regret not having maximized your opportunities. [1]

Reference:

[1] Ibn Al-Qayyim, Al-Fawaid, 315