Firsttimefather, not sure if you check out the other boards here; the "parent/sibling/inlaw" board has a thread about exactly your question:
https://bpdfamily.com/message_board/index.php?topic=350261.0It sounds like whatever the relationship of the pwBPD (romantic partner, parent, sibling, child, etc), it's not uncommon for there to be a "dry erase board" mentality about the pwBPD's hurtfulness in the past.
Emotional immaturity, as you're thinking, is probably a big part of it. Emotionally mature people have tools and skills to deal with shame and guilt about hurting others. Emotionally immature people don't have healthy ways to cope with how bad it feels to have been hurtful, and employ more primitive mechanisms to not feel the shame.
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And hey... I get it about the feeling of "slipping". We want to be better, to do better, and yet when we're under immense stress, we find ourselves leaning on old habits for maybe a quick hit of feeling OK. We have a smoke, or a drink, or (me) waste time online, or whatever, because life is hard and it's just true that we don't always do what our higher values would indicate. What's important, though, that I see you doing, is recognizing that you did something outside of your values (having a smoke). You're not "forgetting" you did it, or blaming someone else for "making you smoke", or minimizing it, or rationalizing "it was OK because". You're facing it as real, acknowledging it, and deciding how you want to move forward.
That's an excellent example of using more mature tools to face shame or guilt.