The speaker looks back at a relationship that has soured so completely that the old fears now seem almost laughable. Once, the thought of life without this person felt like the worst thing imaginable. Now the reverse is true: staying with them is the real hell. The speaker can't even bear to hear their name, which shows how far the feeling has moved from longing to something close to revulsion.
The poem is very short, and that brevity does a lot of work. There's no explanation of what went wrong, no blame, no argument, just the bare fact of the change. The closing question, "When did we come to this?", is the emotional center. It sounds weary and bewildered rather than angry, as if the speaker can't pinpoint the moment things broke. The mood is bitter and sad, with a note of grief for what the relationship used to be. The small slip in the second line ("Now it'd hell") also gives it a raw, unpolished feel, like a thought written down in a hurry.