It happens far too often, and yes it is maddening.

I was diagnosed bipolar for a number of years, and on medication for it, but when I reacted badly to every medication they tried, they re-evaluated me. Of course I'm going to react badly when it's the wrong medication, given for the wrong condition. I am not bipolar. I have recurring major depression, borderline PD, and PTSD. The mood stabilizers they were giving me were too strong. And as for always being sleepy--it turns out that's nothing to do with mental health, even with the conditions I do have. If I'm sleeping a lot, it's not because I'm depressed. I'm not unmotivated or trying to escape from life. I am certainly not lazy. What it is, is sleep apnea. I can't stay asleep long enough for my body to benefit from the rest, before I partially wake up (not aware of doing so) because I'm not breathing. The sleep study showed I was waking up more than 60 times an hour, which means I'm not fully asleep for even as long as a minute. Therefore even after sleeping all night, I'm going to be sleepy the next day. Not my fault, but how many times have I been told to get my fat, lazy you-know-what out of bed; the sun is up, it's a pretty day, and I've slept long enough, but if I'm that depressed maybe I should see my therapist about why I don't want to get up?

My sister went through something similar. She had been hit by a car and sustained a permanent head injury. When certain behaviors started showing up, she was told she had schizophrenia and put on medication, which she refused to take. She did some research on her own and then suggested to her therapist that narcolepsy as a result of the head injury might be the problem instead. The therapist (who keep in mind is not a doctor and has no medical training) responded, "Oh, that's ridiculous. Take your medicine." Guess what, it turned out she had narcolepsy as a result of the head injury, and has never been schizophrenic.

Diagnoses do get changed. It happens all the time. What I find most maddening is when the patient is right, and the professionals aren't listening because we don't have a degree, so what do we know?