It's not that simple, even for the doctor. I've had about five doctors tell me that I have ADHD which always led to amphetamine based prescriptions. That generally helped with the attention issues, which was my biggest concern.
The problem is that the mood issues were always just... "me". It just seemed like an aspect of mild depression that was always manageable. The daily internal dialogue would go: "My mood is absolutely awful right now. I have zero energy and feel depressed. But, this is just a fleeting feeling that should go away in an hour. I'll just deal with it. Coffee should help a bit for now, at least." In other words, I would swing between moods similar to bipolar II.
Turns out that's the wrong way to look at it. Some major life events caused me to re-evaluate many things, so I took a major vacation by myself without contact to the world to gain a lot more self insight. Eventually that led to me asking my psychiatrist about bipolar/schizophrenia as a possible cause of my problems. (Some article on hacker news on mental health actually led to bringing up the specific questions.)
When I brought it up, he even looked at me like I was trying to completely self-diagnose and initially shrugged it off. Technically, yes it's self-diagnosis, but it led to a more-correct diagnosis, but only after sending me to a research program that gave me a full psych evaluation. One of the doctors even mentioned multiple times "you're the perfect candidate for this study..." Go figure..
Mild aside - Similar doubt happens when I ask about lead poisoning as a possible attention/mood root. I used to shoot rifles competitively for years without taking care of washing my hands and such, so it's not a completely awful theory. Every doctor I brought it up to looked at me like an idiot and would just send me to get a blood test as if that's even a valid test. Still no answer there.
(edit - editing)