A toxic, mutually-destructive relationship is one in which both people feel as though the other person is always interfering with their best ideas, skills, initiatives, and characteristics. Rather than both people thriving, each feels as though the other one is actively working against them, trying to sabotage everything they are trying to be and do. Over time, each person shrinks—becomes smaller and less than they were, rather than bigger and more.
When two people significantly diminish each other, just by being their best selves, they should not often be in the same room together, much less in a relationship with one another. They need to distance themselves from one another and live their own lives.