You can grieve while someone is still alive: during a terminal diagnosis, during dementia, during a slow decline. The loss has already started and you're living inside it.
It's confusing because you're mourning and caregiving at once, sometimes feeling guilty for grieving someone who's in the next room.
Anticipatory grief doesn't spend your grief in advance. Many people are surprised by how much is left afterwards. That's normal, not a sign you did it wrong.
What helps: saying the important things now, taking photos and recordings you may want later, and allowing yourself breaks that aren't about them.