I like the Shark Cage metaphor:
"The concept of the Shark Cage begins with the idea that the world is like an ocean, filled with fish of all colors and sizes, and there are also predatory sharks which are dangerous. In the ocean, the woman needs a ‘shark cage’ to protect her from predators, but allow friendly fish to pass through.
We aren’t born with our shark cages, our caregivers and others we come into contact with support the construction of our shark cage. Each bar of the cage represents a boundary or a basic human right, such as the right to not be touched, not to be shouted at or called names. Once the bars are in place, the cage provides a protective barrier making it difficult for sharks to get close enough to take a bite.
However, not everyone has had caregivers who knew how to help their child build a sturdy cage, and many women have shark cages with missing bars or a weak alarm system that needs some work. Importantly, the metaphor emphasises that it is not the person who is deficient, but it is their cage. It follows then that the skill of maintaining a robust cage (boundaries) is something that can be learned and refined."
I grew up in a very abusive and volatile home. Drugs, violence, police raids, psychological and sexual abuse were a normal and sometimes daily occurrence. When I met ex dp he was a like a protector come to save me from the big bad world. Sure, he was a stoner and had selfish, lazy tendencies but he never raised his voice, wasn't violent, didn't smoke crack upon waking and most importantly he protected me from other men. I honestly thought I'd found a good one.
It was mumsnet and reading about different types of abuse that finally made me realise that he wasn't in fact, a good one. He's a lying, selfish, lazy covert narc that almost broke me. I compare him to a black hole, beautiful and mysterious from a distance, but get too close and he'll suck you in and crush you. There absolutely were signs there in the beginning, but I thought his good points outweighed the bad. As time went on the bad points eventually outweighed the good and now the gloves have come off and he's gone full arsehole.
I know my example is extreme but I think it applies to a lot of bad relationships. So many of us grew up in dysfunctional households or had bad relationships modeled to us so we didn't see those red flags for what they were.