Your vocabulary always impresses, and then some days I see it is rife with additional meaning which usually makes me fret.
When I talk about things that make me uncomfortable I retreat into metaphor that's ambiguous to the point of becoming meaningless or making so many precise word choices that it becomes impossible to understand my actual meaning without a dictionary, and even then...
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This week in therapy, my therapist said that they'd never worked with someone who so thoroughly disengaged with their feelings when talking about their feelings.
To more clearly answer your question, a mechanic repairs things objectively. It may take some testing, but eventually the cause of an issue will be determined and resolved.
While a science, mental health is far more subjective and relies (too much imo) on the subject for diagnosis. To borrow your metaphor for a moment, picture the smallest automotive mind you know. Now imagine if your mechanic could only diagnose the problem on a car by talking with your small automotive mind. Now imagine most of the cars the mechanic works on are hamstrung in a similar way. Welcome to mental health.
There are two principal consequences of this. One, there's generally an assumption on the part of provider that you're not qualified to judge the severity of something like substance use/abuse and two the patient is put in the position of weighing productive use of session time against being honest.
In intake I'm honest about two things. One, I grew up around alcoholism and two, I handled the first major depressive episode I experienced in university by drinking. (To contextualize this, there is a period of months in my life that I have no real memory of because of alcohol abuse.) While I drink now again, I was a teetotaler for around two years after this part of my life, which I consider relevant because something obscene like 90% of alcoholics can't stay sober one year.
Because of my honesty, I'm subject to periodic enquiries into my drinking in session that while I intellectually know come from a desire to help, make me feel infantilized and burns a session for me because it pisses me the fuck off. I also have to have things alcohol related hijacked by discussions of alcoholism. For example, last Valentine's Day was my first solo one in a long time. Instead of dealing with that like a big boy, I bought a bunch of bourbon and got crunk as heck. I reported this in session without being asked because I was disappointed in myself for not even trying to face the travail. How was this handled? Like I relapsed. Still don't know why I wouldn't face my fear, eventually I just lied so that session could be productive again.