Hi Alexandra,
I'm wondering whether the nervousness is in anticipating of being ridden when there is a generalized back ache
With that in mind, I would probably try a formula called Du Huo Ji Sheng Tang - that is, a formula to Expel Wind Cold while being tonifying.
Chiropractic may help here, too, or acupuncture
If you only have access to single western herbs, then something warming, aromatic, and circulation enhancing like Cinnamon is called for. DHJST has it in there, as does another formula called Gui Zhi Tang that you may be able to source. It has an additional muscle relaxant known as Peony (Bai Shao) that will provide a more complete impact
Hope this helps you out!
Steve
Steve