Honestly this lady sounds like a complete vulture. Who swoops in TWO WEEKS after your mother passes away and starts hounding you for her horse? You may have to push back with her just so she knows she can't walk all over you. People like that find pushback to be terrifying because they're used to steamrollering everyone else.
If you have to be blunt and say "Excuse me, I lost my MOTHER. I am grieving and I am in pain, and I need you to respect that. This horse was a huge part of my mother's life and I am not willing to let her go just like that. I am not interested in leasing her out to anyone, and may never be ready. Should I see an opportunity where I would welcome you working with her, I will definitely let you know, but please consider the matter closed until then."