It's the idea that somehow magic chemistry can address the machinery of the mind and get it right that defies logic.
There
is "machinery of the mind". While there is plenty of mystery in its operation, some of it
is understood at least well enough to provide a suitable fix for some malfunctions. Yes, it's a tragedy that too many guess at a cure, and don't measure the cause to compute a cure.
True bi-polar disorder does exist, and is indeed "merely" an imbalance of certain chemicals. Just as drug users (legit or not) introduce chemicals into their brains to induce certain consequences, some of what's already normally there can get out of whack and have tragic results. In my friend's case, things settled right down once properly diagnosed and proper levels of lithium were administered (with some side effects like shaking) and later replaced by other balancing medicines.
What confuses the issue is that a fairly straightforward chemical explanation manifests itself in the form of a major psychological screwup which snowballs rapidly into all kinds of mental, relationship and physical problems. Few people can rationally comprehend and act on the clinical facts of the problem - especially when suffering deep depression, paranoia, and other major symptoms.
If your computer's power source very gradually starts upping the voltage provided to the processor, the symptoms would be very bizzare indeed. Diagnosis and correction could be pretty simple ... but fixing hundreds of damaged files, if not the whole machine (after you threw it at the wall in sheer frustration), would be terribly difficult. Ditto bipolar disorder: the cure exists, once the problem is diagnosed and properly & consistently treated, but cleaning up the collateral damage is hard indeed.
All that said, there are plenty of people who simply don't understand it, and cannot differentiate between complex symptoms induced by simple causes vs. inconvenient but normal behavior vs. (shall we say) just being incurably messed up. Too many are quick to blame that mysterious "bipolar disorder" when it isn't, giving the disease and its cure a bad rap. I'm sure there are true cases of ADD which can be properly controlled by Ritalin; unfortunately, Ritalin also manages to pacify normal childhood enthusiasm & distraction into a mind-numbed "good little student" so it gets wildly over-prescribed by social pressure. It's all complex enough that most people just throw up their hands and declare the whole issue a bunch of BS - a pity, resulting in too many being mis-treated and subsequently screwed up for life.
Thumper, sorry to hear it came to this. I personally know how it can wreck an otherwise good relationship. A pity it can't be worked out; you can't make it work if she won't get treatment and clean up the social mess.
Best of luck to you.