Here, a speaker looks back at what they once believed about someone and admits those beliefs were wrong. The first four lines begin with "I thought" and list hopes: that this person was right, kind, honest, and would never cause tears. The way the lines repeat makes them sound like a sad habit, as if the speaker keeps going over what they trusted.
Then the poem turns to "I know." Now the speaker faces facts: the person isn't theirs, seems to be doing fine, and understands exactly what the speaker is going through. That last part suggests the other person is aware of the pain and carries on anyway.
The final lines hold the poem's real point. Despite the disappointment, the speaker still loves them. The short, simple rhymes keep the tone plain and unguarded, so the mood is quiet heartbreak mixed with stubborn, helpless affection. Nothing is resolved, and that's what makes it feel honest.