I wouldn't know, but I would go back to the doctor and explain it's longevity and your frustration with it and your desire to really attack this problem seriously. Ask about other possible diagnoses.

If it is an infection ask the doctor if it would be wise (because it keeps coming back) and safe to use stronger and longer antibiotic courses. I like to aid antibiotics whenever I have an infection by using alternative methods as well. Oregano oil, colloidal silver, garlic are good infection fighters. Other things can boost your immunity such as vitamin-c, zinc, elderberry, echinacea, etc. Staying away from excess sugar and alcohol is a good idea anytime you need a strong immune system.

Finally, after any course of antibiotics it is important to replenish your probiotics. Probiotics are the beneficial bacteria in your colon that aid in digestion, absorption, assimilation, and also keeping bad bacteria in check and aiding immunity. Without them digestion is poor (which is key to health) and bad bacteria are without check. You can replenish these probiotics by eating plain yogurt for several weeks and/or purchasing a probiotic supplement from a health store. If you use a supplement find one that is enteric coated, so that the bacteria can survive your stomach acid.

I've had NSU recently, actually tested positive for Chlamydia, treatment is the same Doxycline for a week or single dose of Azithromycin. Reading your story Danny May (I know it was some time ago) - it just seemed really obvious to me that you got re-infected by your girlfriend, as there was no mention of her treatment, and of course it would keep on happening. At least this reply should help others if you (hopefully) got it sorted out already.