Well, where
else would it be? I thought I had DDNOS for a long time and was misdiagnosed with it for years. I have dealt with structural internal systems since I was 4, due to prolonged solitary confinement in infancy/early childhood. Your experiences exist, regardless of the diagnosis.
BPD and DID can be extremely similar in presentation, and I know several people with BPD that frame their experiences through the lens of structural dissociation. It's a disorder with both instability of identity and heavy dissociative features, so the overlap isn't insignificant.
I wound up having schizoid, which is on the same spectrum as schizoaffective - I think your psych is correct to take DID off the table because of this as schizo-spectrum disorders meet the exclusion criteria of DID (it can't be caused by something else).
A majority of people with symptoms of structural dissociation will wind up having schizo or personality disorders instead. And even for DID, there's no evidence to suggest that they have "different people" in their heads. Neurotypicals converge identity
components in adulthood and those with these disorders don't. Mix in maladaptive fantasy and dissociation and constructs and you get parts/alters.
Even while diagnosed as DDNOS I've never considered that I have different human beings in my mind, my constructs are just that - constructions I embody to cope with external circumstances. The diagnostic component is less relevant to me because my experiences exist regardless of diagnosis, and I am fine with that. If it's something I made up to function, which I suspect it is, it's still legitimate to
me.