Three short lines carry a lot here. The speaker says they like being happy, and the sentence is so plain it almost sounds surprised, as if happiness is something they are still getting used to. Then comes the turn: this happiness is "the total opposite" of how they felt with someone else, the "you" of the last line.
We don't learn who that person is or what went wrong. The poem suggests a past relationship, though it never says so outright. What we do get is the contrast. The speaker doesn't describe the old unhappiness at all. Leaving it unexplained makes it feel heavy, like something not worth going back over.
The mood is quiet and a little defiant. There's relief in it, and maybe a hint of quiet pointedness, since the final words aim straight at the other person. The missing apostrophe in "Its" and the casual tone make it sound like a private thought, honest and unpolished.