I know what you mean. I used to ride Lacey english and basically she'd spook at everything, rear, buck, throw her head and basically just be a jerk. Then I started riding her western and the difference was almost instantaneous. We haven't had a huge bucking/rearing/backing up/head throwing argument in at least 3 months (not including bareback but that was only backing up). She's also practically spook free now.
I think in Lacey's case, she just prefers to be ridden on a loose rein and to have her head (because she'll start regressing as soon as I pick up contact, and she's in a bitless bridle so no teeth issues). In her case too, she's been an english horse most of her life when she wasn't being a pasture puff) so there is a bunch of western things for her to learn- neck reining, and jogging for a few so I don't get bored (like I thought I would) because I'm teaching her to do stuff.
Don't give up completely though! She might just be plateau-ing in her learning for the moment and she's going to be totally better in a day or week or two. It took me at least 6 months to finally ride Lacey western after continuously having problems (which she still has, just not as bad) so don't give up too quickly.
Have you tried free jumping her? What does she do then?
Sorry for writing so much too...