Forgiveness is meant for the victim and not the perpetrator. So, the short answer is yes, one can forgive and it not be harmful. And if one is in a relationship with a toxic person then it can be difficult to move on. Manipulation is key to their success of having possession over you. If you have family and friends that can support you through this time, please tell them what has been going on and what you should do in case the worst were to happen.

I suggest you start writing down a list of reasons why you are leaving them. What they have said and done to you. And just keep writing as you ride out the waves of the breakup. When it is over, burn the piece of paper as a way to claim back your life and put them in your past. Then while you continue with journaling your thoughts and actions, write down all the positives in yourself and the kind of happiness you want to see form into your life.

I hope this helps, sorry I rambled a bit.