I'm so sorry - many of us unfortunately recognise how you feel and there simply isn't anything anyone can say that'll make you feel better.
My dad died almost 22 years ago - suddenly, unexpectedly, and just 3 months after retiring. He had so many plans and was full of excitement about what he was going to do next - and they all came to nothing. My dad, despite being extremely kind and considerate, was a very reserved person and it was only in the last few years before he died that I felt we were truly starting to become closer, get to know each other better and develop an adult relationship. I really valued that process - and it was cut short just like that.
Unfortunately, I don't have a very good relationship with my mum. I feel like I lost the only parent who genuinely cared about me, and therefore, despite what many people would consider a long passage of time, I still feel his death acutely. There are still times when just for a moment I imagine he's still alive, e.g. just after waking up, maybe if I've dreamt about him, before realising with a lurch that of course he's not.
As you say, it's so incredibly hard to think of all the things he never knew - like knowing grandchildren. Perhaps selfishly, I'm also sometimes inclined to think that various aspects of my life might have been different had he been around for longer because he'd have almost certainly offered me a level of emotional support that my mum just can't (or won't) which perhaps would have given me the confidence to make different (better) choices. But overall, it's just this huge stomach churning feeling that you've lost someone so close with no real explanation - and how it doesn't feel "fair" (maybe that's selfish too ?). I suppose it's pointless but I often find myself asking (to no-one in particular) why he had to die when he did ..... but then I suspect anyone who's lost someone when it wasn't "their time" (however you define that) must ask the same thing.
I really don't know what the answer is - well, the answer is there is no answer. I sometimes feel deliberately bringing up old memories can be very upsetting, even if they're happy IYKWIM. But at other times I panic because I can't remember in "enough" detail.
It's really hard because the people right at the forefront of my life now never knew him so I can't talk about him really in any meaningful way. For example, he means nothing to my DP. My mum never seems to want to talk about him much except in passing and older more distant relatives who knew him are mostly gone now themselves. I guess my only advice would be to cherish your memories of him and take every opportunity to chat and laugh about him with people who knew him as well so you're not dealing with all your feelings of pain, hurt, disbelief and maybe anger entirely on your own. I hope you have people you can share this with who also knew him.