By the end of this poem, how does the person feel about his or her decision?

Presumably the speaker of this poem is Robert Frost himself, as he acknowledged in some of his public readings. He is thinking about a career decision he made many years ago. He had to make a choice between seeking a secure profession which would enable him to live in comfort or to devote himself to poetry and live very simply close to nature. Many aspiring artists are confronted with that choice fairly early in life. They don't know whether to trust their instincts and go ahead with the pursuit of their dreams, or whether to play it safe and get a "day job," so to speak. It is hard to do both. Frost seems to have felt it was impossible for him. The two roads went off in different directions, and as he says in the poem::

Two roads diverged in a yellow wood,
And sorry I could not travel both
And be one traveler, long I stood...

Frost chose to devote his life to his poetry. Looking back, as he does in the poem, he is still not sure if he made the right choice, because he cannot know what would have happened if he had taken the other "road." No doubt he would have continued writing poetry. But since his life would have been so different, he would not have written any of the same poems, such as "Stopping By Woods on a Snowy Evening." His published poetry often reflects a simple, rural life. 

Frost does not appear to regret the choice he made, but he sounds wistful in the last stanza of "The Road Not Taken," as if he wishes he knew what his alternate life would have been like if he had taken the other road. Many people feel this way as they grow older. There are so many crossroads in life and so many hard choices to make. When he says:

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

The sigh is actually represented in the poem by a break and a repetition of the word "I."

Two roads diverged in a wood, and I—

I took the one less traveled by...

The "sigh" can be interpreted to mean that he does not necessarily regret the choice he made but that he recalls all the hardships and privations his decision cost him over the years. We get glimpses of his independent lifestyle in many of his poems, such as "Mending Wall" and "Two Tramps in Mud Time."