Hi,

Sounds like the texture of the painted plaster isn't smooth and so
will never match any filled areas.

If the original paint on plaster was rollered on (ie bit like orange
peel) might be worth trying different paint and rollers on samples to
blend it in filled areas.

If a very small area I'd try something like smoothover then sanding
with a random orbital sander to get the whole lot flat.

Otherwise it's lining paper, a skim of plaster, or a textured paint.

cheers,
Pete.