There is no easy answer to your question of why now, at age 44. Our bodies change over time and this includes our resistance to infection, both local in the nose and sinuses and systemic with our immune system. It might also be that you were not before exposed to the infectious agent, or the magnitude of the exposure, that has caused your problem for the past few months. It is very likely that your ENT is correct – that the improved drainage following surgery may allow the infection to clear. A deviated septum can cause a lot more problems than one might expect from a simple change in anatomy, including sleep disturbance and impaired exercise tolerance.
If you have any doubts, seek a second opinion, either from an ENT unrelated professionally to your current ENT, or from an infectious disease specialist.