Lo nuestro is a somewhat dramatic way to refer to your romantic relationship when talking to your pareja. Although this expression may be too telenovelera for non-ironic usage, it could get a laugh in the right circumstances.
- Lo nuestro se acabó. We’re done. (I’m ending our relationship right now.)
- Se enteró de lo nuestro. She found out about us.
- Lo nuestro no puede ser. We could never work out as a couple. The expression un amor imposible suggests an external impediment, for example, both of you are already married to other people.
Do note that the article is lo and not el. And while we’re on the subject, relationship is relación.
Mi peor es nada, my better-than-nothing, and mi media naranja, my half orange, are two cutsie ways to refer to your better half, your ball-and-chain, your old lady/man.