Divorcing a narcissist

When you first met your partner, you had already heard about narcissist or manipulative abuser. Most of the time you may not realise that your partners’ behaviour is narcissistic, and it was not your situation.
Step by step you felt emotionally down. Tears, endless discussions and sleepless nights have replaced your fairy tale.
How to realise that this incredible person at the beginning of your relationship, the one you loved so much, turns into being a monster?