I can only speak about my own experiences with my mom. Others might be different. Perhaps the fact that you have gotten therapy has helped you understand what an apology is. Maybe an undiagnosed BPD would react differently to an apology.
Over the years I discovered that apologizing to my mom was the worst thing I could do. For myself, if I apologized to keep the peace, I was told that I was being insincere and was a cold person. If I apologized because I really was serious and thought that was the right thing to do, I was still told that I was insincere and was a cold person. Years later my mom would throw up the apology as proof that I was a bad daughter and awful person. After all, I had admitted it with the apology.
For those who suffer from this disorder, how has being diagnosed helped you to relate to others?