Practicing cornering doesn't help if you don't know what your doing. Everyone thinks they corner like Sagan and just don't lean enough but often its deeper than that.
I'm also a bad cornerer and I worked on it for months and it didn't help. I would get in more sketchy situations than guys that would rail a lot tighter and faster than me. Always thought it was a comfort thing, that I had to get used to railing corners at speed. It wasn't until I had others watch me and point out my positioning on the bike was waay too forward did I actually improve. I could still probably rail it tighter though but holding wheels cornering on descents is not an issue.