I had the same experience for 5 years, and I took, on different occasions, Levaquin, a Z-pack, and Augmentin. Awesome antiobiotics indeed, they killed every pathogen in my body except several in my sinuses, which grew back immediately.
According to Mayo Clinic research, sinus infections don't follow the regular model of tissue infection. I've borne out that research with a sinus flush that washes out the infected mucus, and I am on my 5th year of clear sinuses. All those saline rinses and sprays you tried didn't work because gravity prevents the saline or spray from reaching the upper sinuses, where chronic infections often reside. The Flip-Turn uses gravity, rather than fighting it.
http://www.medhelp.org/user_journals/show/2322
The Flip-Turn Sinus Flush is mildly risky, because you have to bend over to do it, preferably in a shower, but you can also do it outside on soft ground.
Many people are having success with the Reverse Flush (Chapter 13) which appears to be less risky, and as one forum member related, has been used by his ENT for 35 years. I always prefer doctor-approved stuff.
Good luck, persistence rules.