When poet marked the first road for another day, what did he doubt?

In The Road Not Taken, Frost doubts that he will ever return to this spot and face the same decision, knowing how "way leads on to way." This suggests that the narrator has enough life experience to realize that the future is an endless series of branching decisions that rarely (if ever) lead back to precisely the same set of choices. Even so, the narrator "kept the first for another day", hoping that the future would provide the chance to try the other path, and fulfill his wish to "travel both." The poem ends on a wistful note, stating that in the future, he "shall be telling this with a sigh", and that his decision to take "the one less traveled by...has made all the difference." Does he doubt that he made the right choice? Perhaps; it is not clear. The reader is left to determine whether the outcome of his choice was good or bad. In either case, it is impossible to know what would have happened if the other path were taken - and this, in the end, is what the poem is about.

The Road Not Taken, by Robert Frost, is essentially a poem which metaphorically discusses the theme of how the choices we make in life define us. The narrator comes across two roads in a yellow wood; the roads are both fair and, although the narrator would like to experience both, he needs to choose. In the end, he chooses the one which seems less worn. We could take this as an implication that the narrator tends to make different choices from the majority of people and chooses paths which are less traveled. 
He walks down the main road for a while, but wonders about the other road, thinking to himself that he will one day come back and travel down that one as well. However, here we encounter the narrator's doubt; knowing that one step in one direction leads to many more, he also knows that with every step he is walking further and further from that other road, and will probably never have the possibility to go back.  

And both that morning equally lay
In leaves no step had trodden black.
Oh, I kept the first for another day!
Yet knowing how way leads on to way,
I doubted if I should ever come back.

 
 

Frost, the author of and 'traveler' in the poem doubted that he would ever be back to the spot to take the other road. It is a common thought for all of us at some point. We have a decision to make, a "direction" to decide upon, and wish that we could make more than one choice. As Frost did in "The Road Not Taken," we mentally save the other option for another time but, like Frost, know that it is unlikely that we will ever return to this spot in our lives to actually take that other road.  This poem is a great example of the author's humanness and of the common emotions that his words always bring forth. It also shows his appreciation of the world around him, as he wishes he could take both roads and is prompted to try to remember the other for another time.