Blessed are the flexible for they shall not be bent out of shape
You do not however want the sort of determination implied by phrases like "don't give up on your dreams." The world of startups is so unpredictable that you need to be able to modify your dreams on the move. The best metaphor I've found for the combination of determination and flexibility you need is a running back. He's determined to get downfield, but at any given moment he may need to go sideways or even backwards to get there.