Hi Consider,
Sounds like you have been through a lot with this.
Are the doctors diagnosing the infection actually culturing and making slides (to examine under a microscope) to determine the cause of infection?
You might also think about any lubricants and/or condoms, or anything else having contact with your body. (No need to respond out loud here.) Many people have adverse reactions to the many products available on the market.
Flagyl is a very potent drug. Many experience side-effects when taking Flagyl.
You could try oral probiotics. The oral probiotics will not interfere with an exam or a culture. Oral probiotics are something gynecologists have suggested to their clients for the past 20 years. Yet, please also return to your doctor for further work-up. While probiotics many help with a part of the issue, it's not an adequate approach for you at this time. You need further medical help.
I am reticent to make any other suggestions, as it's unclear what's going on and it's important your doctor do an exam and obtain a culture.
I'd suggest seeing one gynecologist and returning/calling when the treatment does not work out. By sticking with one, the doctor can work through diagnostic differentials/possibilities.
Don't let this go, as your pelvic health may suffer longer effects/complications, depending upon the cause.
As Wiix has mentioned, it's also possible your partner may need to be checked out by a physician or at a clinic, as well.
DejaVu