Seeing the bright side of every toxic relationship

If one is able to alter their thoughts about any relationship, or behavior of partner/friend/whatever, how do decide whether to keep the relationship alive vs. letting it go if it’s “objectively” not a great situation. If you’re always trying seeing the best of everything, and creating your own happiness, at what point do you question or decide whether the relationship is one you want to keep? We shouldn’t kid ourselves that a relationship is good just because we’ve managed our thoughts, right?