Unfortunately, as hard as this is, it's also reality.
My aunt lost her husband 4 years ago and it was a genuinely awful and traumatic loss. Nobody expected her to bounce back quickly. But 4 years on, she's still in the cloud. She still can't find any joy in anything really, and the grief is so profound she still can't face even trying to manage a lot of "life admin" things (husband used to do it all) and her daughter and sister (my DM) are still dealing with all of that. Won't try professional help, peer support, anything new at all. My cousin is an only child and it's a lot for her. Our DC are more or less the same age (late teens/early 20s) and bless them, they've tried so hard with their granny to involve her in things, affirm their love for her and wanting her there at family events etc. They take her out for coffee, take her to the shops, help her at home, etc. But nothing family do or say really seems to help.
It's got to the point though where the people who are left are starting to feel unloved and rejected by her. They are in a way starting to go through their own loss/grief process for the loving, engaged mother/grandmother they had before who doesn't appear to be returning. It's not her fault, everybody realises this, and everyone is still very kind and compassionate and helpful to her. But I don't think they quite feel the same level of empathy and compassion any more and there is some resentment creeping in that she just won't try anything or change anything at all. The really hard thing about prolonged and complicated grief like this is that it will often lead to more loss and grief as it begins to erode other relationships around the sufferer.
I've been asked to try to do something, as I'm a psychologist by background, although this isn't my specialism. But the only thing that's going to help is for her to want help and to want this to change - she can't be forced to be ready. I have a feeling there are also what we call secondary gains going on here - as long as she remains in the cloud, others will be kind and supportive, they'll do the hard things for her, etc. If she exits the grief cloud, she'll have to start facing these things, so it's insulating her, in a way. I usually find with all long term chronically unwell and very stuck people there's a secondary gain somewhere, and that's so difficult to untangle and needs such careful and compassionate handling, even by a therapist or psychologist. It involves challenging your patient to confront their shadow, so to speak, and you really do need to have a strong therapeutic relationship and a lot of trust to do it successfully. Family and laypeople can't be expected to try to help someone through that just with love, empathy and kindness.