(an entry for the Prosperous Writer Prompt)
I suspect many writers are introverts. They gain their energy from within themselves: thinking, reflecting, and writing. That said, I wonder if the caution and lack of ambition for some writers is simply because it is not a natural part of their personality? It may take some learned extroverted personality traits to be enterprising.
If in order to become enterprising, it requires undertaking a project that will change you and cause you to grow, my suggestion is finding a writer friend that is more extroverted. He or she will be naturally enthusiastic, encouraging, and a great nudge!
Additionally, the introverted writer has to believe in him/herself. Outside circumstances cannot be the final say in success. There will be wins and losses. A writer has to weather the rejections, losses, disappointments ... and keep going ... not spiral down into themselves. The creative energy within has to be projected beyond inner thoughts to think big! Yup. These writers need their imagination to go beyond the written page and into the future of possibility.
Writers have to stop saying almost apologetically, "I'm just a writer", like a mom may say, "Oh, I'm a stay-at-home mom". Enterprising will come when the writer treats his or her craft as valuable. They will then progress to making a living at it.