I don't know why she thinks its impossible but the way I see it, something is holding her back. That happened to me a lot. I would encourage her to keep believing until she believes again. Tell her positive thoughts and that if it's a dream, NOTHING is impossible. She needs to push on and never give up even when it seems impossible. People who dream big chase their dreams endlessly until they are what they want to be and if you give up, you will be back from the start. You may fall and think "i'll never be an actress" but it doesn't mean She isn't one.
This is just part of the answer, it really depends on what type of person she is. But in general, talk to her about things that'll make her happy, things that will convince her to chase her dreams. Or have her watch motivational videos on Youtube.