Sandy, I am not an expert by any means but thought I would comment on what you have written.
I would have thought that 57 was quite late to be perimenopausal (sure someone will put me right!), I also know that Metformin can be gfiven to regulate a menstrual cycle as it is often given to women with polycystic ovary syndrome.
It sounds very much like you are going through the menopause now and possibly may have been going through it for a while, hence the bleeding happening more often..3 times in seven days doesn't sound like individual cycles, more like one which is sporadic during the week.
Obviously we are not doctors and you seem to be under the care of specialists but a hysterectomy is a big step to go through if you can be treated some other way. I think you really need to go back to your Gp and Gynae (are you in the UK?) and get a proper explanantion for what is going on, they must have suggested a reason as to why you are experiencing this.
Have you had your hormone levels checked, that would indicate if you are in menopause and at least then you would be certain.
Hope all goes well and you get the answers you need.