Probably you're too young so far to have read much about grief or death and dying, Haze, but there was a very famous book by a woman named Elizabeth Kubler Ross more than two decades ago that first gave names to the stages of grief, which she may have listed in a different order from the way I list them here:

Sadness/depression - self-explanatory
Anger - ditto
Bargaining - thoughts like "Maybe if I take good enough care of her and make sure she sees this doctor, she won't die."
Denial - denying that someone is as sick as she/he really is or wondering if perhaps they didn't really die
Acceptance - where finally you feel at peace

People who are dying and people who have lost loved ones go in and out of these phases at different times during the grief process. Actually, people who are divorcing or going through other types of losses experience these stages, too.

Anger like you're having today is just natural and expected. You just have to make sure you don't take it out on others who don't deserve that anger. But you do have to go though that and feel it, just like you have to feel and express the depression and sadness, too. It's even natural to feel angry at times with the person who's dying or has died.

The good things about these phases is that they change, so you don't usually stick in one too long. Eventually, if you allow yourself to move through the other phases, you reach acceptance and mostly stay there, but you still feel sadness periodically, too. I'm trying to keep this all in mind as we deal with my sister's terminal illness, too, as is she.