Who’s fault is it when a relationship fails?
Two words for you: the man’s.
Usually.

Note: a relationship failure is something different than letting a
relationship decline and die, or just breaking up with a girl,
because you’ve lost interest or other things in your life have taken
precedence or she’s let herself go and is no longer meeting your
requirements for someone you’ll keep around in a relationship capacity.
In that case, while a female observer would still consider it a
“failed relationship”, from the standpoint of the male it probably
isn’t (unless it’s a case of her letting herself go... then, maybe
it is, if she was what you wanted before that but stopped being it
after it). In that case, it’s just a relationship that didn’t work out.
When I say “failed relationship” here, what I’m talking about is a relationship
that you really wanted to work out... but it went belly up anyway.
Is it ever the woman’s fault?
Yes, sometimes. Rarely. In
the case where the relationship is a female dominated one because the
man has yielded to her the role of captain aboard the good ship
Relation... in that case, he is following
her lead, and the direction the relationship goes is up to her.
Even then though, I’m torn... because most women who lead don’t want to do it, and resent men who make them do it as
weak. If I’m the officer of a military unit and I hand over my command
to one of my subordinates, who then goes on to lead us right smack into
a disaster, am I absolved of all blame, or do I still take some too?
The large majority of the time, across most kinds of
relationships, it’s the fault of you, the man – and nearly always, it’s
a failure of leadership that causes the relationship to fold.