Because they are seriously psychologically damaged.
Because they wanted to know everything about you at the start of the relationship and now they know how to use that information to cause psychological damage to you.
Sure when you first told them personal stuff they seemed sooo understanding, just so you would share more with them.
The more knowledge they have the more power they have over you and that is exactly what they use on you because now they know what will break down your self, making you vulnerable and easier to manipulate. Your broken self will abandon you feeling nothing but negativity and emptiness inside. That’s how they can make you hate yourself. They bring up your past and put more fuel on it causing a reaction out of you. A negative reaction released within you = weak minded. They know how to bring your vibration down to their level as a temporary relief to their own unresolved trauma which is not your responsibility
They are paranoid about everything and everything they view is toxic and dysfunctional.
That kind of situation is not something your SELF wants to vibe with so as a result your self then fades from within you. Your inner self will only take one step toward you when you take one step back from the toxic person.