"I respect knowledge of the psyche," she tells Teen People in its December-January issue, on newsstands Friday. "I would be a therapist if I weren't an entertainer."
As for how else she deals with the pressure of fame, rumors about her marriage and other people's expectations of her, Simpson, 25, says she writes religiously in her diary.
"It's a way for me to know myself, my problems and my issues. If I don't do this, I'll push everything back until I explode."
As for stories about trouble between Jessica and her husband Nick Lachey, she says: "Hopefully mine and Nick's story will continue for the rest of our lives, like what we vowed, through sickness and in health."
Simpson also says she often turns to her little sister, Ashlee Simpson, for comfort. "Ashlee and I are closer than we've ever been in our lives."
Jessica and Ashlee posed for their first-ever photo shoot together for the Teen People cover. The younger sister tells the magazine that she goes to Jessica for help as well – especially now that she feels comfortable as an artist on her own.
Ashlee also talks about her own tough times after her SNL lip-synch fiasco and her getting booed at the Orange Bowl in January.
"After the Orange Bowl, I went to my dressing room and dropped to the floor and started screaming. I was bawling like, 'God, I cannot believe this is happening to me again.'"
After making a glitch-free return to SNL, she says she's learned to trust herself.
"Yeah I messed up in front of everybody," she says, adding: "I love to sing. It's a joy to me. I don't do it for anyone else – I do it for me."