I am really sorry that the drugs didnt work as expected, but with surgery an option you are not at the end of the road yet so I really hope you have a good outcome if you opt for that.
I do have a story to tell about the 18 year old horse my sister and I have. Navicular diagnosis at 8, vet advised shoe, keep him going for another few years, not really many other options. Did some research, changed vets, went barefoot, sound horse inside 6 months and he has never been lame on his fronts again.
However, the lack of work whilst we were sorting his feet lead to a very odd lameness behind. Not always, just sometimes. Blocks, hock xrays etc found nothing, eventually found kissing spin. Now we lovingly refer to this horse as the grey giraffe as that is his prefered way of being. We just wanted to ride and have fun, no dressage, no contact, had a nice time but we were passengers. We were told that unless we turned him the right way up with correct riding, to a contact, lifting his back (therefore opening the spinal processes) that there wasnt anything that could be done other than very major surgery. So we learnt to ride, we used a bungee to help as we werent good enough to ask consitantly then and all you had to do was look at the xrays to see why he had to. So he improved, we improved and he is 10 years later doing more than he was then.
Take from that what you will, but I choose to ride my horses as 'correctly' as I can, and if I ride on the buckle they do not go inside out and hollow (unless they are noseying at interesting things!).