So, I've got this idea for my main story that I'm working on, and I'm thinking it's going to be the main character facing her demons, after the suicide of her cousin where she found her body, I'd like for her to show her grief, a year later which is when the story takes place, and her fear of forgetting her cousin, but have her family and her boyfriend that acceptance doesn't mean forgetting, but saying "She's gone, but it's okay." Any tips
A method I always thought worked when done well was the sudden reminder. You see something, go to a location or meet someone that reminds you viscerally of the person you lost and it’s a punch in the gut. But since time has passed you notice that it hurts just a little bit less and you know you will be ok. Many people will have lost someone in some way too, so it’s relatable and doesn’t need to be hammered home.
How you handle the grief a character is feeling will depend on the character. Is she resilient, or does she take a long time to bounce back from things? Did she develop some PTSD from seeing what she saw? If so, is she the type to work through traumas and try to process her feelings, or does she sink into depression and stop taking care of herself when tragedy strikes? Of course, if a year has gone by, then your character will probably have healed some regardless of her initial reaction. It likely won't be on her mind all the time anymore and it won't hurt as much. You still need to decide certain things about your character. Is she a sentimentalist and keeps mementos and pictures around? If so, there's probably something on the wall or a shelf that brings back memories every time she sees it. She might also be in the habit of playing a song that reminds her of her cousin. When she thinks about her cousin, does she remember the good times, or does she dwell on the loss? Either might be true at different times depending on her mood, of course. Another important question to ask yourself is what their relationship was like? If they were close and spent a lot of time together, or if they confided in each other frequently, then her cousin might have left a hole that hasn't been filled yet. She might wish that she had someone to talk to the same way. Also, does your character believe in an afterlife? If so, maybe she talks to her cousin on occasion and hopes her cousin can hear her. She might even imagine what her cousin would say and try to work out a problem she's having by taking advice she thinks her cousin would give. Figure out some of these things about your character, and you'll have a much easier time addressing the grief. Good luck!
Another thing to consider is "the demons" you mention. That she has demons suggests she has a dark side, or that something terrorizes her. Which of these most closely describes what your character is going through?
