The parting of the ways that the poet is faced with symbolizes a choice, but what the poet fails to realize at the time is that once a decision is made one way or the other, it will be irrevocable. "Oh, I kept the first for another day!" is his excuse to himself for not thinking the situation over more deeply and taking the other path, since it is clear from the ending of the poem that the poet regrets the way he has chosen to go.

In the literal case of a fork in the road, we can of course retrace our steps and take the other path if we choose. However, even then,

....knowing how way leads on to way,   
I doubted if I should ever come back.

One thing leads on to another, and retracing our steps is at best difficult. When the road is used as a metaphor for life, as it is being used here, the retracing is not just difficult but impossible, since the poet cannot go back in time. His regret at what has proven to be a bad decision will go on forever:

I shall be telling this with a sigh    
Somewhere ages and ages hence

He will not return because he cannot, no matter how much he may wish to.