... but then I find SSM's absolute rejection of all suggestions of change equally mystifying.
I likewise find that comment mystifying. Unless you mean my resistance to trying something for the 5th time, etc., that has not worked in the past. But I'm not singling you out. Almost everyone here seems to me (my impression, not a fact necessarily) to think that I just fell off the turnip truck, just realized I have a problem, and am turning up here for my first round of advice. Rather, I've spent 15 years dealing with this issue with many therapists, who also did not just fall of their turnip trucks.
The length of time that I've been in this situation has skewed my perspective on what a normal relationship is like, and so it has been helpful to see what other people think is normal in a relationship. But as for advice as to what to do, I'm a harder luck case than the people who turn up here with a fresh problem.