Paint ceiling or skimming required
Wow!  That is a great job of removing texture.

You have (at least) three choices:


Add texture&mdash;maybe one you like more than the old one&mdash;which greatly helps hide joints, fasteners, and other imperfections.
Skim coat and sand.  Making it perfectly flat may take a newbie quite a bit of effort.  Expect at least four coats.  Use non-diffuse work lights well away from center:  the best would be next to the ceiling at the far edge from where you are working to readily reveal shadows of non-flat features.
Paint and hope for the best.  If it is ugly, maybe you could keep the curtains closed and engineer the light so the ceiling isn't noticeable.