Demi Lovato is mourning the loss of their grandmother. The ‘Skyscraper’ singer took to their Instagram Story this weekend to reveal their grandparent, Sue Hart, sadly passed away. They captioned a snap of them together: “This hurts so f****** bad. “I’ll miss you and our phone calls so much Grandma. I miss you already @merryhearts.” The 29-year-old pop star also revealed that their “hero” grandma saved three people’s lives by donating her organs. They added in another Story: “She saved three lives donating her organs. “She’s a hero.” Demi’s mom Dianna penned the following tribute: “My heart is …