In the venerable 'well actually' StackOverflow tradition of not answering the question: as a developer, don't consciously invest too much of your time trying to become a better developer.

Instead, set aside time to become a better marketer. If you're a solo developer you will need to get the word out to investors and customers about yourself and what you can do. Working for someone else, you'll additionally have to sell your ideas to your co-workers and management.

Even in just asking this question, you are demonstrating motivation to do your best as you create. However, your creations will typically be far better served by your ability to promote them.