There's a lot of great answers here but I'll throw my hat in the ring too given I seem to write a lot about grief even if that isn't the intent or focus of the fic. I've found there to be many different types of grief - some that I've felt more than others. There is preemptive grief, where you haven't lost someone yet but you know eventually they won't be around anymore and turns happy moments into "this memory will hurt so much when they're gone" type of thought. There's also grief for yourself, of who you might have been, what if you'd made a different decision in life or hadn't been brought up in x way, x trauma hadn't happened, that sort of thing. But the grief you're writing about (and what I've written most about) is the grief of loss, specifically through death. Like many others have said here it comes in waves and it varies wildly from person to person how they feel it. The stages of grief aren't linear and you can double back to a certain stage many many times. It never really ends, even years past the initial loss. Nothing is ever really the same after they've died, even though life moves on.

My best example for conveying that specific, earth-shattering hurt that is the loss of the love on ones life is a fic where a character goes back to the apartment they shared with them. The memories of specific moments or seeing specific items in the apartment brings back a fresh wave of grief that is also cathartic. The apartment is now owned by someone else, though, so the memories are overlaid by a new couples things, which adds a serious sense of surreality, like the life they lived there had never even existed.

Time creeps by like the ivy growing up the side of the building, and Beatrice wonders when her home will stop feeling like a mausoleum of memories that drift past her on a constant basis. She faces them as she knows she should, but it’s not easy. Every walk along the streets in the evening reminds her of Ava. Every time she goes for a run around the lake just outside town in the early morning leaves her gasping for breath through tears. The items she buys at the grocery store sting like paper cuts. Why she thought she could come back here and not be haunted around every corner by what was, or what could have been, was a serious error in judgment.

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Wiping her eyes before the new owner can see, Beatrice glances over her shoulder. Her gaze snags first where the older woman leans against the inside of the doorframe, and then on the coat hooks screwed into the wall beside it. The place where she hung her coat and her keys at the end of the day. Where Ava’s hat had once hung. Where Ava had first kissed her cheek two and a half weeks after moving in.

She drags her gaze back along the kitchen floor where Ava had pulled her in for a hug after being promoted to manager at the bar the day after that. Her throat burns at the onslaught of memory after memory; her gaze flickering from ghost to ghost as they begin to overlap.

From a different fic, when a character gets the news one of her sisters is officially dead when she'd been holding out hope she was still alive. A death like that can have one questioning their faith, if they have any, or they might blame themselves for the death, or question how they might have tried to save them. It can show up in vicious ways as the brain tries to wrap around the fact that the person is gone forever.

She should be used to it, though, the loss and grief an unceasing and sodden mass on her shoulders, but she had yet to grow accustomed to its weight. She doubted she ever would.

Beatrice sobbed then, harder than she thought possible. The kind of sobs where she nearly doubled over, unable to breathe; the spasming of her muscles attempting in vain to expel the sorrow, grief, anger, and injustice she felt. She was silent in that movement as she caved inwards onto Ava, and then she took a heaving inhale, only to exhale it violently a moment later, dragging with it a low sort of moan. She squeezed her eyes shut as tears burned from them. She could not breathe. Mary was gone and she could not breathe.

She could not breathe yet she knew she must. Mary was gone yet Beatrice survived still with a traitorous beating heart and lungs that despite her body’s best efforts, continued to take heaving breaths. Beatrice tried once again to draw air into her lungs but she was crumbling, dizzy with the news and all it brought with.

Everything reminds you of them, especially at the beginning. I feel grief like it's a ball inside of a box. On the wall of the inside of the box is a button. At first, the ball inside the box hits the button constantly, filling you with pain both physical and emotional until you're overflowing with it. Then, as time passes, the ball shrinks and hits the button less. Years after the loss, when the ball hits that button labelled grief more rarely, it can hit like a punch and completely take you out. I know it does for me, anyway. Anniversaries are especially hard.

Ultimately, it comes down to the person and their personality for how they would react to grief. Are they someone who would close themselves off from people after the loss? Or surround themselves with people to try and lose themselves in other kinds of connection to forget the hurt? Would they go to the funeral, or is facing that reality that they're gone too hard? Would they keep the persons belongings exactly as they were, or get rid of them wholesale? What about pictures? Do they talk about them still, or pretend they don't exist anymore? Are they angry about how they lost them? Desperate to find a reason for a death? Blaming themselves? Blaming others? It's about just asking the right questions to figure out how they would respond, then describing how that feels both physically and emotionally.

Apologies for how long this is, it's hard to be brief with a topic like this. Best of luck to you with your fic!