Your parent died and someone told you it was the natural order. They probably meant it kindly. What it does is tell you your grief is not supposed to be this big.
Most people outlive their parents. That makes a parent's death the most common serious bereavement there is, and the most minimized. The people around you have a script ready: they were old, they had a good run, it was their time, at least you had them this long. Every one of those lines quietly sets a limit on how much you are allowed to feel, and for how long.
There is no limit. A parent's death reorders your life. It moves you up a generation. It takes away the person who knew you first and longest. This page says the plain version of what happens, and what actually helps.