Passing of Baby Cousin

I (27F) say "baby" cousin however he was just shy of his 20th birthday. We are 1st cousins who grew up together I'd say most of the time. We are related through our respective father's side (my father is his father's older brother). I remember (albeit barely) when his mother was pregnant with him and his evolution from curious baby to brave and fearless bug-collecting kindergartner. I remember the first time he screamed "I hate you!" at me, why I don't remember, but I do remember feeling like I got kicked in the chest and it affected me so deeply I went to my parents at age 13 and apologized for carelessly throwing that phrase at them. He used to teach me to play video games which I am very bad at. I remember being very impressed that he was so coordinated and intelligent at such a young age. I remember the first time he showed me a message he'd received from a girl in his class. Half of it was about his "sage green eyes" (he is mixed race but could pass for white, while I am fully black) and I told him not to get too big a head just because he was fortunate enough to inherit some form of his dad's hazel green eyes. I remember him being so big and tall before he was diagnosed. Then he got rail thin and his pretty curls fell out and I was told by our grandma he was still being a good sport about it all. Then remission, a return to normalcy, I've never wished for someone to have a normal ass teenage/early adulthood so much. His curls grew back looser but still a beautiful mousey brown like when he was a kid. Then a return, a decline, and now we're here.

I feel simultaneously like it's not real, I've lost one or two fingers, and that I'm not entitled to grieve. After all, my aunt and uncle lost a son, I've lost a cousin. Plus I only saw him twice when he was sick. I hate myself for it. I am an ICU nurse. I've seen people sick before, it's what I do. However when it came to him this need for avoidance and privacy for my aunt who I figured must have had a million things going on prevailed. I didn't want to acknowledge it, I didn't want to see him how I see my patients. I mean I did at the end, now, for the first time, the ICU is repulsive to me. I don't know what to say to my aunt or the rest of my family. Or what I should do. To add to the problem this is probably the first time someone relatively close to me has passed and he is now the youngest person I know to have passed. I'm not sure how I should feel. This is so ironic. I talk to patients all day about grief but I don't pretend to understand what they're going through, even with this it probably doesn't compare to losing a parent or a child. But when it comes to me I feel like I need that stupid brochure we give people.