She said:" Because I'd risen so quickly, I was frightened I was going to sink quickly too.
"At the same time, I felt I didn't really deserve all the success I'd had. Although I still worry about that, I'm getting more relaxed now."
The 'I Dreamed A Dream' hitmaker was tipped to win 'Britain's Got Talent' but lost out to dance troupe Diversity and Susan - who checked into rehab to be treated for exhaustion immediately after the show ended - admits she expected to be forgotten soon afterwards.
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She added in an interview with the Daily Telegraph newspaper: "I just needed to go home and rest. That's when I thought it was all over.
"I thought no one would take me seriously afterwards. They'd just assume I was crazy."
Susan gets recognised a lot when she goes out and is amazed at how quickly her identity sweeps through a crowd.
She explained: "If I stayed in one place for long enough, people would start to say, 'there's Susan Boyle'. It's like Chinese whispers. And the next moment you've got people asking for autographs and taking photos."