I'm sorry to hear your sad news. I think things will feel a bit better after the funeral, but you will still feel sad for a long time. Let your feelings dictate how you move on.
However, if there is some project that you could do 'in honour of' your grandfather or grandparents - fundraising for a charity connected with them, planting flowers in your garden (or in a pot) that they loved, having a memory corner in your room, setting their photos in order with captions (though that can feel very raw) - I personally find that it helps.
I was very close to my grandmother - she died thirty years ago now - but my feeling has faded and I now think of her without sadness but with gratitude for the wonderful person she was. It helps that I feel sure I'll see her again in heaven - I can even 'talk to her'.
The loss of my Mum 15 months ago is still very painful. Even just today I went into a charity shop, spotted a lovely china teacup, and thought 'I'll get that for Mum'. What a twinge when I remembered.