It sounds like you are in a frustrating cycle. How long has this been going on? I gathered from your post that you are post menopausal, how many years?
A decrease in estrogen can lead to atrophy of the vaginal tissue. This can also lead to a decrease in normal vaginal flora, which alters the pH of the vagina. Normal vaginal pH should be between 3.5 and 4.5 and we often see this elevated in post menopausal women. This can in turn lead to increase risk for infection and this includes the urinary tract as well as the vagina.
So things you can do include all of the good suggestions Rosa posted for you and also supportive local treatment for the vaginal tissue. Estriol locally, most likely what your physician gave you, can help as well as vitamin E suppositories and lactobacillus suppositories.
I suggest visiting a qualified Naturopathic Physician in your area for some of these treatment options. Check www.naturopathic.org to find one.