Oh, I kept the first for another day! Yet knowing how way leads on to way, I doubted if I should ever come back (13-15)
We already know that the speaker wishes he could take both paths, but here he comes back and tells us so with an exclamation point. He knows, though, he won't be back to take the first path. With his adventurous nature, one path leads to another, and it rarely gets him back to where he started. This pushes us into the metaphorical level of this poem – unlike a walk in the woods, it's hard in life to start over once we've made a choice.
Two roads diverged in a wood, and I-- I took the one less traveled by (18-19)
Even though it was a little unclear in the rest of the poem that one road was more or less traveled than the other, our speaker thinks that, in his future, he'll remember taking a less traveled path. This confirms his love for exploration, going where few others have gone.