The speaker looks back at a quiet change in themselves. They can't say exactly when it happened, but at some point they stopped lying about "having no one else." The wording is a little tricky. It suggests the speaker used to claim that one person was all they had, perhaps to someone else or perhaps to themselves, and that this claim wasn't true. Now they've let go of the lie.
The poem doesn't say why the lie was told. Maybe it was a way of seeming loyal, or of hiding other relationships, or of making one bond feel more important than it was. The subtitle, "just him," hints at that single-person focus, though the poem leaves the details open.
The mood is quiet and reflective, with a hint of relief mixed with uncertainty. The speaker's "I don't know when" makes the shift feel gradual and honest, like a truth that simply settled in over time.