Is there a fine line between 'wishful thinking' and optimism?

You would have to define what your meaning of "wishful thinking" is. There are things like flying or lifting up a house with one finger (etc) that would go in the "wishful thinking" category but all the things that a human being has achieved, can be achieved by any of us. It's all in the matter of "practice". Because practice makes perfect. I always believe to aim higher than you could ever wish for. Because usually the problem is that most people aim to low and surpass there goals to easily.