"I'm not as guarded as you would think. I should probably be more guarded. I trust a lot of people, actually," the actress and singer, 29, told Ryan Seacrest on his KIIS-FM radio show Tuesday morning.
Not that she's over feeling disappointed in Mayer, who later apologized. "That's truly how I felt and still feel," she said. "Somebody that you have cared about for a long time, you just hope they can keep the intimate things intimate."
On a lighter note, Simpson – whose new VH1 show, The Price of Beauty, premieres March 15 – introduced her new single, "Who We Are," and spoke warmly of Smashing Pumpkins frontman Billy Corgan, who worked on it with her. There has been speculation that the two are in a budding romantic relationship, though Simpson recently said they were just friends.
"I can't find good enough words to speak about Billy," Simpson said. "He's a very, very dear friend. For him to give his time and his talents and share them with me, it was a great experience. And I hope we can do more music together."
"Who We Are" is the theme song for The Price of Beauty, in which Simpson travels the world looking at what is considered beautiful in various cultures. The show was inspired by her weight controversy last year, when, she said, "if I put on 10 pounds, it was the end of the world."
Looking back on those days, Simpson said then-boyfriend Tony Romo helped her through the worst of it.
"Tony was so great and so supportive and never ever made me feel like I wasn't sexy," she said. "I was embarrassed by what the world was saying, but I still had my friends and loved ones and family who appreciated me."