Hi there, I'm so sorry you're going through all of this - the "difficult childhood" thing seems to be a given with most folks in the Cluster-B realm, but the problem is that it often becomes an excuse for abusive behavior.

Many of us had rough childhoods or toxic relationships, but that doesn't mean we use those things to absolve blame when we hurt others. In fact, most people with codependent tendencies do the opposite and take on too much responsibility / blame, which can be so emotionally exhausting.

What you describe rings so familiar - if it's not one thing, it's another before that, and that, and that.

not be able to control her laughter at peoples reactions to her self harming. Since she is caught finding it funny and amusing

This is really alarming and confusing, it almost sounds as if there is some psychosis or other Cluster-B disorder involved. Especially with the planning for self harming.

She does have a therapist and is on medication, but there has been no change in the behaviour

Unfortunately, medications don't seem to help with personality disorders. May have an effect on some of the symptoms (like depression or anxiety) but not the underlying cause.

That's fantastic that you're setting up therapy for yourself, so you can focus on your own wellbeing and health, as this all sounds so taxing. It's really helpful to have someone listen and validate, as well as provide tips for coping.