It's not my fault and It's my fault are the opposing extremes on the continuum of The Blame Game.
In abusive relationships, people stay in their role pretty consistently. In toxic relationships or when people are frequently in power struggles, they take turns jumping between positions. Conflict erupts when they both want the position of It's not my fault.
When people believe they are flawed, worthless, or dispensable, which shows up in feelings of depression, anxiety or anger, they are stuck in believing It's my fault. They have countless stories of what's been said or done to them by others. It gets twisted into the evidence they use to keep themselves convinced that it's true.
However, there's often a long path of unhealthy relationships with people who just don't play nice in the sandbox with others -- they're not well! Even with that evidence, they grab onto one tiny thread and twist their lives around it: if all these people think it's ok to treat me like crap, they must be right -- It's my fault.
With some insight, they often recognize the source of their beliefs. Even then, however, the road to un-twisting their thoughts is a tough one. Regardless, it's worth the effort to replace those distortions with new beliefs, the truth.
An important note: It's not my fault people actually have an incredibly deep belief that they are flawed at their core, so they hide it by anchoring themselves in this category. If they convince you to accept blame for all the relationship problems, they hope you will feel so lucky to be loved by them, that you will stay with them. Their goal is to not be abandoned.
Living on the continuum of blame is where we discover that we are indeed human. Being human means that we voluntarily visit the full spectrum of blame regularly.