Hi AFFA,
Welcome

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From what you've learned about BPD have you had any situations where you felt like BPD might be causing a fight? What areas are you having difficulties with boundaries?
Boundaries are not so much about putting your foot down. Boundaries are like fences. They are there to keep the good in and keep the bad out. Another way of looking at it is what are you responsibilities and what are your SO's responsibilities and then don't let those get confused.
One way to determine what is good and what is bad, is to first determine what your values are. So, in all of your relationships, not just romantic, what are you values? Our workshop on
Boundaries might help you understand this a little more.