I know how you feel... My best friend in the world, my dad, passed away a year and a half ago when I was 23. The last time I saw him was on Christmas and he died five days later unexpectedly, I tried contacting him multiple times previous to see how he was and he never replied. I regret so deeply for not going to see him again... But my mom was there and I thought all was ok. He was my everything. I'm very close to my mom as well but my dad and me were "soul mates". The only thing that keeps me sane is knowing the one thing he hated the most was seeing me upset. Still at times I can't help myself.
I don't have my dad's horse but I do have his saddle. I know if someone were encroaching on that I'd in no uncertain terms let them know it's not going to happen. It may possibly never fit one of my horses, but it will never be for sale. Tell her you are not interested in any way shape or form of leasing, selling, or otherwise sharing your moms horse. Nine months seems like a long time but wait it out and you won't have to deal with her anymore. Don't cause drama or stress, but just turn down her offers. Best of luck to you and if you need anyone to talk too please feel free to PM me. Nobody can understand the loss unless they have gone through it. I still see my dad in my dreams and I cry when I wake up and realize it's not for real, he's still gone.