I want to say first off: I am sorry that happened to you. It was not your fault. You've done brilliantly to get to this point. You've survived them.
Moving forward, it seems to be quite common for the 40's and the onset of menopause to be a catalyst for turning around and looking at your parents and asking WTF. I think it's partly because the hormonal change means that women are less pliant, less obliging, less willing to offer care to their own detriment. It's time to take stock and ask yourself what you want the next chapter of your life to look like. I had a reckoning with my father a long time ago, but the major struggle with my mother is happening now (I'm a bit older than you). I recently did 6 months of therapy for the first time, after years of knowing it was inevitable but not being ready to face it. So what you're feeling is understandable, and it's normal, really.
With regards to the medical records, I would ask this question: Are you SURE that your parents can give you the information you feel you need? Because yes, they were the adults who were there at the time, but it's entirely possible that they may have agreed to things they didn't understand, they may not have listened, they may have forgotten, they may have dealt with doctors who assumed consent and didn't inform properly. As you say, it was the 80's, and record keeping wasn't great.
If all that is keeping you in contact with them is the hope that they might be able to give you information, maybe it's time to re-evaluate if you want it badly enough, and if it will be worth the damage it is doing to you now, because it won't change where you are now.
and FFS stop the duty phone calls. Give yourself a break from them. It's okay to not phone people you don't want to talk to, even if you're related to them. It's allowed.