I don't know if it's this one ( http://www.mangago.me/read-manga/glass_mask/ ) because I didn't read it yet but the description you made seems to be very similar to the synopsis that a friend of mine made when he recomended me the manga.
Found it: Sweet Bitter Love, http://www.mangago.me/read-manga/sweet_bitter_love/
it's completed but not completely scanlated.
I'm looking for a I think it's a Manhwa about a nanny/maid who switches her baby with the lady of the houses baby. The lady of the house is also dead. The nanny then treats her daughter really well because she is now the bosses daughter and the real daughter is treated bad because the nanny just doesn't care. The fake daughter is acting and the real daughter want's to be a actor but is forced to wear hat's and glasses to look ugly so the fake daughter stands out.
I read this years ago and it wasn't completely scanlated so maybe it has been completed.
Thanks for all your help.