The reasons targets of these con men types find it hard to leave is usually because of a trauma bond. There is always something good about the relationship too, something the target is getting out of it. Even if it is only the drama high in an otherwise lonely life.
But these situations are all based on power and control. The abuser wants all the power and all the control. Again, if you listen carefully to how a victim or target speaks, they use terms that express powerlessness, lack of agency, fear of loss... the abuser has honed in on this, usually by inducing fear or low self-esteem.
All a nasty Jedi-type mind trick played usually on lonely and sometimes needy people. We all need companionship and they often hook in on individuals who have successful lives, children, a thriving business, but have little or no companionship. This is why it is hard to leave.
The brain has to be retrained. First of all, the person has to ACCEPT that the abuser is NOT a good or healthy person to have a relationship with, grieve that, then work on themselves and on establishing healthy personal boundaries. But by continuing to allow contact, this perpetuates the misery, The victim MUST see the abuser for what he or she is, and whether we want to call them dangerous, evil, whatever, the one thing that they are is toxic and bad for us.
But THAT is hard to accept at first, and that is because of the addiction to the highs and the lows and whatever is good about the highs. The victim has to understand at a logical level what has been done to them... the programming by the abuser. Usually this is the hardest stage, as cognitive dissonance holds sway in the brain. Here we have two opposing thoughts "But he LOVES me, he left flowers, he is texting and calling and cannot live without me". The other "But he lied to me, he has been shagging other women, he refuses to change his number".
Which one do you think will win? Probably the first one if the abuser's program is left in the brain and the victim is too afraid to hit the acceptance stage of the TRUTH. The second conflicting statement is the TRUTH. He is a liar, a cheat etc... Once that is truly taken onboard, the real work can begin.