but because I've become pessimistic that doctors will find something that works that my ability to detect any improvement is impaired.
Once you get a negative schema (in this case about the efficacy of your meds) you filter will out positive information about you stamina levels (a bit like the comfirmation bias, really).
Of course it could just be the drugs don't work. Trying keeping a graph of your stamina against differing meds: that's a longditudinal undertaking but it's the best way to find out for your self.
As an aside one interpretation of the Hawthorne effect would say that you will feel better for making the graph of your stamina.
Funny thing, psychology.