I fear my own strong personality has made the condition worse
Having the strength to say that will bode well for your relationship with D26

I need to find ways I can be a better mother, remaining calm and learning to diffuse the situation.
Do you find you remain calm in most other relationships? No one can push my buttons like my stepdaughter (23). What I have learned here to help me stay grounded and calm has changed my life. Not just my relationship with SD23, but with everyone. You're in good company here. We walk together and try to learn and grow and develop specific communication and relationship skills.
any conversations towards her becoming more independent often result in horrible arguments.
People with BPD tend to experience profound anxiety about separation. Masterson, an expert on BPD, talks about the catastrophic feelings that separation can elicit in someone with BPD (which he calls abandonment depression). To avoid feeling those catastrophic feelings, a person with BPD is likely to act out.
Are you and your husband able to support one another? It's not uncommon for one spouse to have a different idea about boundaries than the other.
Glad you found the site. There's lots to learn here and wonderful people.
