What makes the narrator doubt whether he should ever come back?

When Robert Frost writes "I doubted if I should ever come back", what he's indicating is doubt that he'll ever be able to return to the position he's currently in. He's comparing his awareness within a situation where an important choice must be made to having to choose a path in the woods. He expresses the seriousness of this choice by saying that he'll likely never be able to return to this moment of power or clarity over these choices after the decision is finally made, and that future important choices will compel further action instead. Frost is illustrating powerlessness over what may happen next, and concludes the poem with a sigh and by saying that he took the path less traveled, which made all the difference. In other words, despite all the challenges there are to making an important decision, Robert Frost expresses that he chose to live a certain way in order to cope with these hardships, and that certain aspects of life are beyond the individual.

The narrator knows how "way leads on to way," and this is why he believes he will not make it back to this particular spot. In the poem, the two roads are symbolic of two choices, and, typically, when we make one decision, that leads to more choices and other decisions, and our next decision leads to new choices and more decisions. The likelihood that one would actually be able to retrace one's steps, so to speak, and somehow return to that one particular decision, having had to make so many since then, is not good. Even if it were possible, would we really want to undo or unmake all the choices we have selected? I doubt it, and so must he. That’s just the way life is.