I'm going to suggest "The Road Not Taken" by Robert Frost. If read literally, the poem is simply about a guy who chooses to go down a road that most people are not going down. The final line of the poem seems to suggest, however, that the speaker might no longer be talking about only a road.
"And that has made all the difference."
It seems as if the speaker is suggesting that choosing to do things his own way has been a very important part of his life. It has made a huge impact. The thing that always gets me about this poem, though, is that the speaker never says if choosing the road less traveled was a good choice. He just says that it made an impact and couldn't be unchosen.
Macbeth relates to the inner journey of that poem because he chose to take a path that few others were in a position to take, or willing to follow: murder and mayhem. Early in the play, Macbeth is struggling with his decision to kill Duncan. He will, then he won't, then Lady Macbeth makes him feel guilty and calls him mean names, and then he will and does kill Duncan. From there Macbeth has to start killing a bunch of other people to secure the throne. Then he is killed for all of it. Macbeth's single act of choosing the road that leads to murder (the road less traveled) has made all the difference. If he doesn't kill Duncan, the play is very short and boring. Macbeth says:
"It will have blood, they say; blood will have blood."
It's like he is admitting that his act has had a domino effect. He made a decision, can't go back on it, and that has made all the difference.