no Bithynia. The shape, mouth and ecology doesn't fit. Looks a bit like Assiminea grayana, but this is an Atlantic species. Perhaps another species within that family or Hydrobiidae is more likely.
Bithynia is freshwater, and as Ton also mentions, the shape and aperture don't appear to match. To me, this looks like a species in the family Hydrobiidae.
You are right, it does not match with Bithynia. However, affinity with Assiminea grayana is rightly suggested. Wish there were a few more specimens for identification.