In Robert Frost's famous poem "The Road Not Taken," the author uses symbolism to express the importance of individuality. The road represents the course of the narrator's life.
In this case, the narrator, reflects that, of the two paths he could have taken in life, he chose to follow "the one less traveled by," and he believes "that has made all the difference." Basically, the narrator reached a point in his life when he was faced with decision to either conform to society and live a "normal" life, or to live his life in way that embraces individuality or an less conventional existence than the first path. The road that fewer people had taken ("was more grassy and wanted wear") was more inviting to the narrator, so he made the choice to follow the path that fewer people had taken. The narrator believes that chosing to follow the norm would have been the wrong decision.
To the point answer to"that has made all the difference" would be that...the poet leaves the interpretetion on the reader that his choosing of that path has made the difference positively or negatively.He just says that to day his condition was just because he had taken that path then it may be good or bad. Can be either ways!!
I thhink the poet is not happy bcoz the choicee he made did not bring about a good difference in his life.throughout the poem there is a negativity like"' sorry'',"doubt","yellow woods" which symbolises old age,"worn out" etc so the poet is regretting he did not take the first road if he had taken the road not taken he would have been happier.