Someone the speaker trusted has drifted away, and the speaker is left to deal with what remains. At first this person was the one who always cared, the one who made the speaker feel chosen. Then things changed, and "first choice" turned into "just an option." The subtitle, "Used," hints at how the speaker now sees the relationship: as something taken from rather than shared.
The feelings are tangled. The speaker misses what was real, admits that being alone is harder than expected, and even tries to be generous by saying "at least you're happy." Yet anger keeps breaking through, and the speaker can't explain why. The sharpest hurt comes near the end, when the other person is with someone else and finds it easy. The speaker apologizes for possibly being a burden, then ends on a bitter note: "Only you did." The mood is sad, raw, and lonely, with the unspoken sense that nobody is listening.