Critical thinking and constructive criticism do not equal mean feedback, nasty behaviour, insulting and hurting your feelings.
No one has the right to abuse you emotionally with an excuse that it is for your good. Even a surgeon uses anaesthesia before operating. When feedback is vicious, the operation can be successful but the patient dies from unbearable pain.
Constructive feedback is digestible only when it is delivered with compassion and kindness. Even the right messages have to be delivered in the right way.
Toxic people are like cancer, being close to them eats you from the inside. They cause emotional psychological damage which often leaves you with permanent scars if you stay too long near them. Their continuous poison can manifest with stress related illnesses. Even worst, it spills over to you. If it is at work, you are likely to bring it back home and hurt your loved ones or the other-way around, if it is at home, it can ruin your working relationships.
When you feel fear to change, consider the cost of damaging your mental and physical health and the damage to your relationships with others!.
A toxic negative person is a one who does not change even after you give her/him a few compassionate chances. Once you conclude that the negativity is a pattern and a character trait (as oppose to correctable mistakes that we all make), that is the time to say GOODBYE and “fire” that person from your life.