I think you have every right to grief. It's horrible losing someone suddenly, and shocking.

But you must come up for air at some point - for your own sake. You never knew this girl beyond text on a screen. You never met. You could have met and you may not even had got on in real life (yes it happens that people get on amazingly well over the internet, then meet and completely clash).

While it doesn't invalidate your feelings or the potential of what could have been, you're torturing yourself thinking "what if?". I've this happen a lot of older men speaking to younger women online who then passed away/ghosted and they grieve for months over it. Then abruptly stopped when they realized they over fantasized it.

You must live in reality.

Perhaps in the future you might be a little more careful about what you comment on people’s posts who are clearly grieving such a fresh traumatic loss.

Fresh traumatic loss? You never met her and only spoke over Instagram for a couple of months. Be real dude. People lose their wives of 50 years, their parents, their siblings, their best friends suddenly. It's disgraceful how you're making this all about you and not her.