I went on to one of my 1.12 worlds but on 1.8.8. I expected it to give me a warning like "This may corrupt your world!" and stop me, but it took me in anyway. I left the game and went back again in 1.12 to see the damage. Some 1.12 stuff was still there, but chunks were missing, and things were invisible or disappeared. I was wondering if there is a chance I can fix this.
Unfortunately, once a world is corrupted, you usually cannot get it back, because the missing parts aren't just not shown, the information in them is completely gone.
Next time you want to switch to an older version, you should probably save a copy of the world in case the world gets corrupted. In general, downgrading is never officially supported, not even just starting an older version in the same directory as a newer version.