What did the poet wish to do when he took the path that he had not been able to choose in "The Road Not Taken" by Robert Frost?

The speaker says he wishes he could go back and take the other road after having chosen one.
"The Road Not Taken" is about a person who comes to a fork in a road and has to choose between two roads. He comments in the beginning that he wishes he could travel both. Since he can only choose one road, the speaker stops and thinks about his choice for a while. He notices one of the roads is grassier than the other one. It is fall, so leaves cover the road.

Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood
And looked down one as far as I could
To where it bent in the undergrowth 

Eventually, the speaker chooses one path, and says he will come back for the other one. He also comments that, given how life usually turns out, he doubts he will ever be back again to take that other path. 

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.

The speaker tells us his decision to take the less-traveled path made all the difference for him. We can assume this means he never went back to try the other one. He seems to be pleased with the less-traveled path, though.
The poem can be seen as a metaphor for life. In life, you cannot make two decisions simultaneously. You might make a decision and then try to go back and make the other choice, but that option is often no longer open to you.