Hello,
I lost a grandparent to cancer several years ago. I went to his house every single weekend when I was growing up, and I made so many memories there with him. Those years spent learning from him were definitely some of the best of my life, so when he passed away, it felt like things just wouldn't be enjoyable anymore.
But eventually things got easier. I was able to watch home movies with him in them and laugh. Just a month or two ago, however, I was talking with my girlfriend about our grandparents that had passed and describing our favorite memories. Then, out of nowhere, I started crying and I had to hug her and just vent it all out. I'm not usually that emotional or comfortable crying around others, so it was a surprise. But it helped to do that and know that I was with someone I trusted and who really loved me.
My advice would be to not pressure yourself to quickly get past your feelings. Everyone does things at their own pace. I would suggest finding someone you really care about and trust, and who you know cares about and trusts you, and just ask to talk about your grandparent. Any topic that comes up is fine, and they're all relevant. Mix in the sad with the happy. Know that opening up to a close friend can only make you closer.
I promise it gets easier.