You're are not naive no
And its not uncommon to doubt a dx when you have more than one and are unsure about what is what.
I mean Idk anything about PTSD, but on the surface my co morbid dxs mimic each other to the untrained eye, and I have to dig beneath them to reveal the truth.
Like if its BPD mood shift, something, or more accurately someone, did or said something to trigger a thought, that triggered a feeling, that triggered an emotional response....
On the other hand, if its completely random, say I wake up wishing I had died in my sleep and my day or week/s progresses along that path and nobody and nothing can untrigger it... Well then that's my BP being a sneaky b!tch.
If I didn't take the time to untangle them I would probably also doubt their validity...
The shytti part about having co morbid dxs is that they tend to feed off each other.
So for instance, my BP kicks starts my OCPD (which I otherwise have a really good handle on) the OCPD feeds the Borderline in me (because of my obsessive negative thought loops), and it can and does turn into one tumultuous hot mess.
With time and introspection I have learned to MOSTLY untangle the BP from the BPD, which means they fuel each other less.
And in turn my OCPD stays mostly manageable.
Like if I can take a step back and realize my BPD is rearing it's ugly head and get a damn grip on myself, then it doesn't kick start, exacerbate or lengthen a BP cycle...
Untangling them is no easy task tho, so don't kick yourself if you realize you suck at it. It takes alot of practice, diligence and patience with yourself.
Give it time, it can't be easy having PTSD and BP.