Your essay is spot on. Loss of an important, valued friendship calls for mourning. Inexplicable loss like the one you describe has a lot in common with sudden death. Maybe it feels worse because the person you lost has a choice.

I lost a good friend, inexplicably. Like you, I reached out to her only to be shut out. Later, her aunt-in-law asked me about this friend a party. It turned out my friend had cut her dead as well, and at the same time. We both took comfort in realizing it was her issue, not something we did.