The English sentence is just fine, here is a quotation in context:
While working madly last month to get ready for the San Francisco EGA Seminar, I maintained the fantasy in my mind that when I returned I would reward myself by sitting all day in my stitchy chair and working leisurely on project after project, sipping tea and munching on chocolate for sustenance.
The thing I'm worried about is whether the Portuguese sentence means the same.
I like that DL gives us sentences that make us think. But this one really is odd. Your example is good, but you have to add more to DL's sentence to make it sound good. I think other possible sentences would be "I keep the fantasy alive" or maybe "she's the keeper of the fantasy", though that last one is stretching it even more.
yes, that's right, it a question of context and habit. Maintain does, of course, mean keep, but it's usual meaning in English is to keep something in running order or condition.