Keep in mind that the Furysword requires Demon Titanite to be upgraded, which is harder to acquire than Twinkling. (can't be bought and must be farmed from the Titanite Demons, most of which do not respawn).Had a thought overnight to build Quelaag's Fury Sword which could help.
My thing is that I've neglected the RPG aspect of this and treated it like Monster Hunter: skills and power ups are cool but not game-making or breaking until high level play. Four Kings is where I reap the whirlwind on this thinking.
Thanks for the tip Noema on Twinkling Titanite.
Also the Furysword is a Dex weapon, I don't know if you have invested in that.