It is very helpful to tell us about yourself, because therein lie the answers for you.

MP is often confused with trochanteric bursitis. Falls from horses can certainly over time create issues with the spine.

http://www.emedicine.com/pmr/topic141.htm

http://www.aafp.org/afp/20000401/2109.html
This website has some nice diagrams. Figure 2B shows the placement
of the lateral femoral nerve which can be compressed under that inguinal ligament, leading to MP. That is the location for best placement of
Lidoderm patches for pain. That spot is the best for me. I've had MP for many years following a C-section.

Trochanteric bursitis and MP often present together, or have similar findings.
Trochanteric bursitis can be treated with steroid injections.

MP was your first symptoms. This suggests to me, that you have mechanical injury as the most likely culprit.

Thyroid and diabetic PN often start in the toes and fingers since they are the farthest away and the smallest nerves. Large nerve attacks are less likely until metabolic issues are far advanced, and you claim that this has been tested for.

B12 deficiency is insidious, and affects everything eventually. And it can mimic MS as well. There are optic neuritis type symptoms similar to MS. So I think you need to get your numbers, and see if your doctor is up to date on understanding B12 replacement etc. Has anyone in your family had vague neuro problems? Or been diagnosed with pernicious anemia?

I think an MRI of the spine and head, would be the next thing to look at.

Figuring out what is going on with PN and other nerve issues, is a complex time consuming process..and many here have found doctors to be not interested
in figuring out the details etc. So in the end many here become their own
advocates and do much research themselves.