I shall be telling this with a sigh Somewhere ages and ages hence (16-17)
Our speaker likes to think about the future. He can imagine himself as someone whose story people are going to want to hear, and the choice in this poem will still be important to him then. But he's being ambiguous about whether or not his future is happy. We don't know what kind of "sigh" he'll be telling his story with – a happy, nostalgic sigh, or a regretful sigh.
I took the one less traveled by, And that has made all the difference. (19-20)
The speaker is staying ambiguous here, but he is stretching the truth a little. He doesn't know if he took the road less traveled or not, really – both paths were covered with fresh leaves. But he thinks that, in the future, he's going to remember the path that he took as the one that was less traveled. We're still not sure whether or not he's happy about this, because all he says is that taking the road he did has made all the difference in his life – we're not sure if this difference is positive or negative.