The speaker is telling an ex, or someone from their past, that they have moved on, but the news comes with a bitter twist. They haven't found someone who treats them well. They've found someone who "can hurt me better." The word "better" does a lot of work here. It sounds like a boast, as if the new person is an improvement, yet what is being improved is the pain.
The tone is hard to pin down. On the surface it's casual, almost offhand, with "I thought I let you know" sounding like a polite update. Underneath, there's sarcasm and wounded pride, and maybe a jab meant to sting the person being addressed: you hurt me, but you weren't even good at it. It could also be read as sad self-awareness, a speaker who keeps choosing pain. The poem doesn't settle which, and that uncertainty leaves a cold, uneasy mood in just one line.