Frost made it clear that the speaker in this poem was himself and that he was talking about career choices he had made at one time in his life. We know that he decided to live a life of austerity and simplicity in order to be able to devote his time to creative writing. We know this because that was the kind of life he actually led and the kind that is described in many of his poems. His choice was similar to that of another great New England writer, Henry David Thoreau, who may have been an inspiration to Frost.
The road not taken must have been a more practical and potentially more lucrative career. We can imagine that road leading to a big city full of opportunities for a man with Frost's brains and talents.
Fortunately, we know for sure that he felt satisfied with the road he had taken because he said so in a letter he wrote to a young girl who asked him that specific question. The letter reads, in part, as follows:
No wonder you were a little puzzled over the end of my Road Not Taken. It was my rather private jest at the expense of those who might think I would yet live to be sorry for the way I had taken in life....I'm not really a very regretful person....
Finger, L. L.: "Frost's 'The Road Not Taken': a 1925 Letter come to Light", American Literature v.50.