Supplements For a Headshaker ?

I have a 10 year old Morgan mare (had her since she was three) that has a head shaking problem. She's been doing it for a few years and it comes off and on depending on the sunlight, wind, and slight rain is really difficult for her, but usually it's not a problem and we do really well in our cles. She's been shown at mainly A rated Morgan breed shows her whole life. This spring/summer has been really bad though, such as when i was trail riding with a friend: we were trotting through a field at the beginning, about to really get going and she all of a sudden started shaking her head so badly she started to trip, so I slowed her to a walk and it got a little better, but we eventually went back to the barn because it was a frustrating situation for all of us (it was not a bug or anything). The next couple rides weren't nearly as bad (in the ring), but still frustrating because she's supposed to be collected/on the bit and it's nearly impossible when she starts shaking. I have been using a nose net for about a month and it's helped some, but I can't do main ring cles with the net (we do Hunt Seat and occasionally some dressage/jumper stuff), so I was wondering if anyone knew of any good supplements that help with head shaking ? I've been looking at SmartBreathe from Smartpak Equine ( http://www.smartpakequine.com/smartbreathe-7178p.aspx?cm_sp=Browse-_-Rev-_-TopPick&cm_vc=TopPick ), but I'm just not sure what to use. It's not a tack discomfort and other possibilities have been ruled out and she does it no matter who's riding her so it's not my riding style. Thanks for any input !