You offered a number of theories. Something that occurred to me is this: maybe the mutual compatibility, which once was there, is no longer there.

If this theory is correct: one can be deeply sad about the loss of a very close relationship that now is gone (for whatever reason(s) that it is gone). The loss hurts, it can be bewildering, one wants to search for the reasons why and for what/who to blame. Still (on this theory), reality is what it is. How should one best face this reality?