Update: L and I talked last night. I have an emotional blindspot that matches up with an emotional vulnerability of hers, I think.
Blindspot: I assumed that if I was truthful and frank about my feelings that she would understand me. And that even if I was clumsy and blundered in the way I expressed myself, if I was genuine and truthful then I would be forgiven. That concept was pretty deeply embedded in my subconscious.
Vulnerability: L needs considerate, thoughtful behavior more than bare emotional honesty. She needed me NOT to be frank about my sudden realization that I could want to live with someone and the incredible excitement I felt. It hurt for her to hear that stuff. What she saw was that I, who had hitherto been careful and tender with her, was suddenly inconsiderate and thoughtless. She expressed it as "You're not who I thought you were. I don't even know you now."
In retrospect I can see her point very well. Too bad I didn't think deeply enough beforehand! Hell, I've only been dating for 9 months after a 30-year monogamous marriage during which I didn't even flirt. My emotional intelligence is way underdeveloped. Primitive and crude. That's just an excuse, of course. It doesn't matter: I screwed up.
So we've taken a break. We'll talk again in 3 weeks. I kind of expect that by then we'll have started moving apart. Maybe not, though. It helps to write this down.