Perhaps what would be helpful is for you to talk to a professional about the abuse you suffered while you were in a relationship with your previous partner so when you next enter into a new relationship you are emotionally stronger and less conflicted.
Individuals who have bpd can have a strong fear of abandonment and rejection your ex sounds like he was trying hard to support you even though you ended your relationship and returned briefly to your abusive partner, it must have been quite difficult for him. It honestly sounds like the relationship that the two of you had was quite dysfunctional for you both and as harsh as it may sound sometimes its better to walk away from the situation for the sake of your own mental wellbeing. Hopefully you will find happiness and peace of mind as you move forward