The curse, eh? Come to think of it... You said the outside and inside were once one.
Two things that ought to be one. The inner and the outer. Two things that are each complete, yet cannot exist without each other. Things which cannot be separated, yet can never be together. Like life... Existence... Light... And shadow... Nothingness... Death. For each to continue to exist, for us to remain ourselves... There is something akin to a boundary -- a wall, a barrier... that allows those two halves of a whole to forever stay apart.
Avoidance is bliss. To meet is to cause each other unhappiness... To cause strife and suffering. Just like it did for us. That may simply be the fate of the world. But... if you meet and can accept that suffering... the two may finally become one.
In the end...
That may be the curse.