I adopted my cat. Rosie, as a package deal with her sister Reece when they were about 6-7 years old. Not too long after, Reese died, and Rosie didn't really know what to do with herself without her sister, who was her only playmate.
She didn't really perk up again until not long after I adopted another kitten to keep her company (which made her "the responsible one" for a change).
So my advice: adopt another companion.