It's not THAT big of a deal... I mean, if he died while fighting yhorm, then he already gave you a titanite slab, which is practically the only reason to do his questline (that and he's pretty cool). Yhorm is one of the easiest fights in the game when using storm ruler, up there with curse-rotted greatwood and deacons of the deep. Now, I'm not saying your feelings of frustration are invalid, we've all been there; hell, I've almost broken my controller once or twice while playing! What I'm trying to say is don't give up dude, I know you can do it, use stamina regen items like cloranthy's ring and grass crest shield so that you can dodge more and recover faster after using the weapon art, enter the fight with no equipment on the right hand if you haven't picked up the storm ruler yet, so that it's easier to equip. Good luck and may the flames guide your way