At the very end of the movie, after Travis gives Betsy a free ride in his cab, his eyes flash in the rearview mirror as an ominous noise sounds. That's it—that's the last we see of Travis.
According to Paul Schrader, the screenwriter, this indicates that we end where we began—with Travis driving around the streets of New York, observing things, stoking his rage. He's not better. He's still crazy, still judging the world from his rear view mirrors.
No one learned a lesson and nothing got better. People mistook Travis for a genuine hero—they didn't realize he almost assassinated a presidential candidate. Travis himself doesn't seem to have learned anything or gotten over his rage. How could he? No one knew what he was really up to in the first place.