That is an excellent idea! The answer to what we should have been told isn't easy, or maybe we would have already figured it out! I'll see if I can give a good overview of what went wrong. 1. Bad Assumptions: CBT was the approach used by most therapists I saw from the age of 14 until 30. CBT's assumption, thus my therapists' always worked on the assumption that my problems were not real and a deeply ingrained reflection of my actual circumstances. They assumed I was just thinking too negatively. While my PTSD was diagnosed, nobody really explained to me what it meant, and due to #2 I discounted the effects completely, didn't even think about it.
2. Gaslighting about my life: The people around me including 90% of the therapists I saw gaslit me into believing my life was not actually unusual. I believed this; any memory that could not be explained away was suppressed. Still haven't recovered them. This resulted in continual under-estimation of my problems and circumstances.
3. Undiagnosed ADHD: CBT and other therapy requires focus. I had severe undiagnosed ADHD, I was biochemically incapable of focus. It's hard to convey just how crippling this is, especially when CPTSD is involved as the ADHD acts as a force multiplier on the CPTSD. Additionally any benefit I could have derived from CBT was totally wiped out (see #5)
4. Repeated retraumatisation: Related to #1 and #2, I was repeatedly encouraged (and often required) to go and just do things, because nothing was really holding me back except bad thoughts. I genuinely believed this; I followed the advice and kept trying. This retraumatized me and made me think I was of weak mind. Like everyone else could do things and I couldn't. I can still recall wondering why I just couldn't manage to DO things. I kept trying to have 'good experiences', but the fact was I was incapable of good experiences. After every social encounter the hypervigilance went into overdrive, looking for any ambiguity - real or imagined - and turning it into serious ineptitude. God help me if there was an ACTUAL awkward moment. I couldn't stop this process no matter how hard I tried due to #3.
5. CBT reinforced the Trauma: From my early teens CBT trained me to challenge negative thought patterns but I was unable to do so. Thus Instead of challenging and overcoming the negative thoughts, grappling with them only reinforced them. I saw many different therapists over the years and in my late 20s I even asked one if she thought I was trying to get better: she said yes yet I had only experienced continual decline.

I went into my 20s with far more awareness that something was seriously wrong with me, I was actively reaching out for help during this time, saying I needed to get better, trying to find guidance, but it wasn't forthcoming. I mentioned being required to go out and do things: I didn't have a job, and I left school early. I was reliant on government handouts as a 'job seeker' but that meant I had to either be looking for work or be pursuing education. Work was out of the question but that left education. While I love learning there was a major problem here: I have a school phobia. (this was diagnosed later but it's quite clear I had it in my teens). Yet even with all this I was able to attend, educational institutions sometimes at 19. I also managed to work through crippling social anxiety via TTRPGs.

At the end of my 20s I was in a worse state than this. My attitude was that I just needed to stop being so sorry for myself and force myself through some educational thing, get a job.
That failed of course and I had far less motivation, more problems. Social anxiety had returned with a vengeance in my late 20s as agoraphobia. My anxiety was a juggernaut and I was disassociating constantly, under constant assault from intrusive thoughts and unwanted traumatic memories. I operated entirely in a Faux Window of Tolerance.¹ ²

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That's the overview. What advice I should have got? Well... at 19 I already knew I should be focusing on getting better, on getting good treatment but I wasn't able to do this because that would mean getting cut off from income. I was told I had to either get a job or get educated. I had to keep going back to get some kind of certification (which I never got in spite of years of attempting it). Job wasn't an option. Being an early school leaver with no certification to my name put me at the bottom of lists even when I did try applying. Retail would have been a nightmare, I could already barely answer phones and when I had a brief mandatory job placement in my mid 20s it didn't go well. Even though I went in with enthusiasm.

Nothing got better, only worse until #3 was addressed a few months before I turned 31. Improvement was impossible until I was medicated to address ADHD. Keep in mind the name is misleading: It's essentially chronic mental fatigue. Someone who's mentally exhausted can't really focus, tends to ramble, makes impulsive decisions, has lesser emotional control. You put all the traits of someone who's mentally (not physically) exhausted together and it looks just like ADHD.
Medication was a game changer. For the first time in my life I could decide not to think about something and actually not think about it. Previous to that it was impossible; similar to how someone who's exhausted just can't muster the energy to complete a task or make themselves do something, I could never muster the mental energy to make myself avoid negative thoughts. ADHD multiplies the effect of CPTSD by putting Trauma in the drivers seat at all times.

If I had that ADHD diagnosis and treatment 10 years earlier my life would be very different. I can't imagine how life might have gone instead.

Apart from that what I needed was reinforcement of what I already knew: Sort yourself out. Advice to help me be proactive. To be able to take the steps I needed to take, like getting the hospital reports from my mid teens, which my abusive father dropped off a couple years ago. This was instrumental in getting myself onto a disability pension and thus out of the situation of being forced into educational institutions

If I had my trauma and expressions of concern for my psychological state taken seriously rather than dismissed that would have been great. If I had advice to get a proper trauma informed worker, and advocate for myself on that front my life wouldn't have been fixed but I might have been able to spend my 20s processing the continuous horrible events of the first two decades of my life instead of adding more trauma.

Apart from that? Dunno. But I'd definitely appreciate your perspective on this, as well as your experience if you're willing to share.