What do I do?

first of all I am sorry for your loss. My father did the same thing to me when my grandmother died, he dismissed my emotions because he thought I was too young at 5 yrs old, I would light a candle every year on her death anniversary to remember her. When I was about your age at 17 he asked me why I still light a candle after all theses years. I told him that she was the only parent I knew up to the age of 5 yrs old, while they (my parents ) was traveling all over she was looking after me, and when they took me away to settle in another country it broke her heart and she died that same year. He broke down, and apologised because he didn't even realise...that I had remembered and felt that way all those years.

This happened to me and I distanced myself from family. Your other option (healthier) is to talk to them about how they’re invalidating you and how you’re allowed to deal with it whichever way works for you. Sorry for your loss 💐