I can't imagine you'll enjoy working elsewhere if you're not happy at Google. If you're an entrepreneur at heart, you'll likely not feel fulfilled until you're working on your own product. I'd say stick around Google (at least for the paycheck) as your mentor suggested until you have an execution plan. Also, if you decide to find another employer in the future, it will be more difficult to explain that you quit because you felt "unfulfilled" (that would scare me if I were considering hiring you); but if you quit to start your own company, that's a commendable act.