My experience is similar. I got up to around 55lbs on overhead press and that's about as good as it gets. Can't seem to get beyond 75lbs on bench press either. Adding more weight doesn't seem to result in any adaptation at all. I struggle just the same or even worse the week after adding weight. If anything, I've mostly gotten less strong rather than more, although I can do more reps, not more weight.
I'm going to try 531 even though I'm hardly strong enough for their calculations. On my 5+ set of squats today (95lbs) I managed 20 squats before the exertion headache hit me. Ugh, that's another limiting factor for me. Felt like a good workout though. Got a good case of the jelly legs.
Female, 5'3", 50, Starting weight: 163lbs. Current weight: 135 (more or less).
I can squat 192.5lbs, press 75lbs and deadlift 210lbs