As others have said, the DSM and current classifications of "mental illnesses" are about as scientific as snake oil salesman giving their pitches.
So you have to take those sort of diagnoses with a grain of salt, unfortunately. You may find yourself in the position of having to seek the truth independently with the evidence that's available, because at the end of the day, all of these professionals are humans like the rest of us and they are not infallible or all knowing.
A lot of my pain could have been prevented if these so called experts had done their jobs properly or had any conception of duty of care. I was misdiagnosed with depression and anxiety as a 13/14 year old following chronic sexual abuse. Due to this error, I was unnecessarily medicated and invalidated, while my physical health festered.
For years, I had severe vitamin deficiencies. Yet, none of these expert doctors would send me off for a blood test until it was far too late. They told me I was depressed and I just hadn't found the right medications yet. I wasted so much of my youth sitting in therapy, being told that I was sad for no reason, when the reality was that I was suffering from physical ailments, autism, and ptsd.
By the time I finally was given a referal for laboratory testing, I had already developed CFS, likely due to my severely weakened immune system being unable to fight off influenza and EPV. I had essentially NO vitamin B12 or D in my blood. They had never seen serology results as shocking as mine before. Because of the chronic B12 insuffiency I likely have permanent nerve damage- but that's still only a theory.
Having those mental health diagnoses on my record ruined me and prevented me from getting any tangible assistance. By the time I had an official diagnosis of Autism, I was about to graduate secondary/high school, so it was too late for any interventions. I had to suffer so much torment and scorn from my teachers and peers, because they thought I was undisciplined and weird due to my sensory overloads, speech deficits lack of eye contact, and fidgety mannerisms.
It was so obvious that I had autism. Yet no one believed it until I had a signed note from a professional confirming it. The culture of authority worship is so ridiculous, you didn't need a psychologist's letter to be able to tell that I had significant speech and social delays.
Doctors continued to gaslight and invalidate me even after I was labeled as suffering from ptsd and autism instead. My CFS is dehabilitating and fucks me over regardless of mood, yet I was still told that my severe fatigue and neurological issues are a result of being depressed. I'm not depressed, I'm just in constant physical pain, which surprise surprise, makes you feel like utter shit!
When I started developing IBS, a gastroenterology specialist told me that I was just stressed and having boyfriend problems, then wouldn't give me any sort of treatment. Time and time again I get treated like I am crazy simply because these people don't have the answers. They don't know how to help someone with complex issues, so they have to accuse people of malingering or mental health issues to avoid liability or fracturing their fragile egos.
I wish they could be diagnosed with psychopathy!
