Great things are only accomplished by those who believe they are capable of greatness.
Those who are happy, and successful in any manner of life believe they are worthy of happiness, and that they are capable and committed to success.
Ambition is good. The desire to improve and grow is perhaps the fundamental reason for being. But unchecked ambition, ambition without a why or end is sure to end in unhappiness. For someone always wanting more there is never enough.
As painful and scary as the decision to act can be, the scariest idea is a scenario where we live our whole lives frozen in inaction because of fear, and thus never even dare greatly enough to attempt a better version of ourselves. Don’t let fear stop you from becoming your happiest self.