Hi Fran, and thanks for getting back to me:
Based on your description of your mare's condition and your answers to my questions, it is most likely that your mare is suffering from a chronic (long-standing) uterine infection. This is not an uncommon scenario in older mares, for as they age the conformation of the vulva changes and becomes "tipped", making the vagina more prone to infection from manure. This infection can then easily pass through the cervix into the uterus and create the conditions that you are observing.
Unfortunately Fran, this type of infection is not something that can be easily cured just by administering some antibiotics. The uterus needs to be manually drained of the pus that is present (and there can often be several quarts.....!), then flushed out with sterile saline and infused with antibiotics. To make matters worse, the procedure usually needs to be repeated several times before the infection is cleared. Once the infection is cleared, it is common practice to literally sew the top two-thirds of the vulva closed (known as a "Caslick's" procedure) as an aid in the prevention of further infection. She will also need to be on a course of oral or injectable antibiotics as well. There is a good chance that your mare has foaled previously, as uterine infections tend to be more common in mares with a foaling history.
Her increased water consumption and weight problems are also associated with the uterine infection, as a tremendous amount of her body's energy is being expended into trying to clear the infection. If she is left untreated, she will continue to lose weight, and there is the possibility of the infection spreading to other parts of her body.
I wish I had an easier answer for you, but unfortunately these infections need to be treated aggressively.
I trust this has helped, and please let me know if you have further questions.
Thank you, XXXXX XXXXX of luck!