I've had this problem. I think I've got a "handle" on it (ha ha), but I need to do more research.
Here's the deal: since puberty, most of the time I'm getting myself off, so that's what I'm used to.
My hand, expected stimulation, and all that. I can't imagine a pussy which can generate that kind of pressure/friction/sensation. I suspect that if a woman
stuck around long enough, I'd learn to cum with
her, but it hasn't happened yet. And as I age, and gain weight, it gets worse.