Well I don't think that is the right way to go about it. Ahhhh, the beauty of horses!!! I think you both need to do a lot of groundwork. That probably isn't what you want to hear, but most trust and bonding comes from the ground. How does she act when you aren't riding her? It sounds to me like she has some anxiety issues. You said you do jumper classes - as in timed? A lot of horses that do timed events, that I have come across anyway, get very anxious when they are ridden because that is all they do.
I applaud you for going every route to try changes, but my opinion is giving her calming supplements is the wrong way to go. It is just trying to put a band aid over whatever issue is present.
However if you would like to try it, it is your horse and I have used "Calm n Cool" on horses that get ancy in trailer rides - rocking the truck and trailer all over. Not sure the exact spelling. It comes in tubes like a wormer and I think it takes like 30-1 hour to kick in. If you want to use that as a trial to see if she calms down at all you could give her some of that a little while before you ride. It won't have a drastic effect, just takes the edge off - so again, if she is anxious enough that won't even help. So I wouldn't look for some miracle supplement that will make her change like night and day.
Good luck :)