Henceforth I ask not good-fortune, I myself am good-fortune, Henceforth I whimper no more, postpone no more, need nothing, Done with indoor complaints, libraries, querulous criticisms, Strong and content I travel the open road.

In other words, the speaker is saying he is no longer asking the world to hand him good fortune. Instead, he is seizing it for himself. He is becoming the change he wants to see. He is feeling good because he finally has gotten over making excuses for not doing what he wants to do.

In his exuberant good spirits, the speaker communicates his sense of well-being and confidence by seeing good fortune as a part of himself, not something separate and out there that he can wait for passively.

It helps, too, to keep in mind that when Whitman uses "I," he is often speaking universally. His "I" represents everyone. Therefore, we he says "I" am good fortune, he is opening the possibility of good fortune up to anyone. We can all fulfill our destinies, he is saying, simply by taking hold of them with confidence. He goes on to list all the obstacles we can set up for ourselves, such as whimpering that it is not possible to achieve our goals, procrastinating, and saying we don't have enough resources. In one grand gesture, the speaker has swept all of that aside to boldly travel "the open road."