It's just fine that Diarmuid can't really put it into words; Bucky can't either. The important part is that he understands what he's getting at.

"It's getting better though," he said after a moment. "I can think about the future. Past what I need to do when I get back."

After they stop HYDRA and dig up everything they can on his past, maybe he can do something with the rest of his life. Make some kind of difference somewhere, maybe. It brings along its own anxieties, but he feels ready to take on the challenges.