The speaker is talking about how risky it is to share a private thought. Some thoughts, they say, are better left unsaid, but the poem quickly moves to a more personal worry: certain thoughts are so special that giving them to the wrong person could be a loss you never recover from. The final line puts it dramatically. The speaker imagines spending an entire life trying to "get that thought back."
What comes through is a mix of caution and vulnerability. The speaker clearly values their inner world and fears that the wrong listener might dismiss it, twist it, or somehow take it away. The idea that a thought can't be retrieved once it's shared is interesting, because it treats thoughts almost like objects you can hand over and never see again.
The mood is quiet, guarded, and a little anxious. The plain wording and casual lowercase style make it feel like a private note, as though the speaker is whispering a secret about secrets.