I'm sure that I'll feel the same reason when D&D 6E is released. But I already have more material than I'll be able to use for many years. I have a tendency to buy whatever cool book is released, but I've finally hit the point where I stopped doing that. I was spending too much money on more content than I can use. When 6e comes out I'll still have plenty of material to get through and I can still buy a lot of what I skipped.
And, besides, there are many other game systems out there that I've not played.
Perhaps to get over your angst, you should take a break from Pathfinder and D&D for that matter and try a completely new system. I find that I much more enjoy buying books for new systems at this time. I don't feel a need to be a completionist, I just get whatever core rules I need and an adventure path. Even a short campaign will give me plenty of hours of enjoyment to make it worth the cost. Then I can just move on to yet another system.
Instead of buying another bestiary or setting book for D&D I would not rather spend my money on a completely new system.