Maybe an analogy (or two) is in order:
I’m not diabetic, but I believe that it’s a “real” illness. Despite the fact that my pancreas does its business efficiently, though, I can mimic some of the effects of diabetes pretty easily: If I eat nothing but chocolate bars and sugared sodas for 24 hours, my blood sugar levels will rise. If I eat nothing at all for 24 hours, my blood sugar level will drop. If I were to do either of those things, could I reasonably say “Well, I’ve experienced extreme blood sugar level fluctuations, and look at me. I’m fine. Diabetes is just some ripoff diagnosis du jour.” The thing is, though, that I am ultimately able to control my blood sugar through reasonable diet and exercise. Not everyone is able to do so. And even people with Type 2 (Adult onset) diabetes, who are able to control their diabetes through diet, exercise, and weight control, have an actual illness - with real risks, like macular degeneration, neuropathy, etc. I can say that I’ve experienced a symptom or two if I’m stupid about my diet, but I sure as heck can’t say it’s not a real illness.
Or maybe one can say “Migraines are just a bunch of hooey. I have headaches, but they don’t disable me.” But I’ve had run-of-the-mill headaches, and I’ve had migraines. They hurt in similar places, but the difference is in degree. I can function with a headache, or even with a hangover. A migraine is a different animal. Like a housecat and a tiger - same idea, totally different degree.
Likewise, “feeling blue” or “having mood swings” are only mildly comparable to being depressed or experiencing bipolar disorder. “Feeling blue” is kind of equivalent to “Man, I’m so upset that my favorite blue jeans are now too tight for me to button.” Being clinically depressed, by comparison, is more like “I’m a fat cow who doesn’t deserve the oxygen I use. There’s only one logical solution to this in my mind.” (And, for bipolar, the manic side of the reaction might be "Well, let’s go shopping. Look! I found 13 new pairs of jeans that fit perfectly! And someone who wants to sleep with me! Yay! Put the jeans and my bar tab on the credit card!) What makes this even more difficult to understand is that (like diabetes,) depressive illnesses can be improved (not cured) with lifestyle changes - improved diet, more exercise, cognitive therapy, etc. Depressive illnesses are very real, but they aren’t like a broken arm or something. There’s no x-ray diagnosis, you can’t just look and tell what degree something is wrong.