We're stuck on semantics here. What you're calling a loss of something, I call a change.
A couple of months ago, I had my last bottle of Belgian-style Quadruppel. It was brewed in 2000. It tasted fan-freaking-tastic. But it was completely different from young Quad! The carbonation was gone, the ester profile had undergone a complete transformation from spicy to winey, and the body had dried out considerably.
Better? I think so. Different? Hell, yeah. Still definitively Belgian? Absolutely no question: yes.
So, no, I don't agree that esters and phenols degrade after 3-5 months. I think the way these flavor precursors interact with other precursors changes over time. Thus, I don't think your beer is suffering from quality issues at all; it's changing. If you don't enjoy it after a certain amount of change - which you've determined happens at a certain point in the space/time continuum - that's perfectly all right! Just knowing that shows that you can overcome the obstacle by drinking beer. Yes, that's the solution I'm recommending - drink your beer when it's at the peak of its flavor.
If you have any left over, send it to me for a thorough, professional analysis. I'll email you the results.