Yet knowing how way leads on to way, I doubted if I should ever come back (lines 14-15)
Despite his efforts to be optimistic and persuade himself that he can take the path he didn't choose on another day, the speaker sees that, realistically, he'll probably never be able to take that path again. There are many decisions in life that just can't be remade. Once you choose, for example, to accept one job offer, you forgo any others. You can try to apply again, but chances are slim a company will want to hire someone who's already turned them down.
I took the one less traveled by, And that has made all the difference (19-20)
These final lines let us know that the speaker's choices were significant. This is not just a choice of which path to walk on a morning stroll, but a choice that determines the direction of the speaker's life. These lines are probably some of the most quoted lines of poetry in this country, and also probably the most misinterpreted. Though they read at first glance to be positive, who's to say that "all the difference" is bad or good? The speaker doesn't even know if he made the right choice yet, because he's forecasting what he'll be saying a long, long time from now and leaving his words ambiguous.