Fear of Forgetting

I have this weird fear, that when a person dies, I will forget them. Not particularly forget that they existed. But forget the little things about them, the memories we made and experiences we shared.
When a loved one dies, the grieving process is slow and long. And I won't go into what the grieving process is, that's a whole other topic. But a very important aspect of grieving is "time." The amount of hours, days, weeks, months, and years it can take to return to normal. A beautiful thing time allows us humans to do is move on, create new memories, and focus on the living people in our lives. But time is exactly what I fear will take my memories away!
I want to be able to remember every afternoon I spent playing cards or Chinese checkers with my grandpa.... And every Easter, Christmas, 4th of July, and birthday we got to celebrate together. But with every holiday and birthday that passes I feel like the old ones with him become more distant, almost foggy. So To keep those memories sharp I must re-live them. Which elicits sadness once more, and often reawakens my grief. So my natural instinct is to stop feeling those sad thoughts so much.
I know it's impossible to forget the person, the physical human being. But I want to remember all the little adventures and conversations we got to have.
How do I do that without being sad about it at the same time?
I'm not sure...