Hey Anonymous 644277, welcome to the group -- glad you decided to reach out.
Glad you have a therapist, the more support you have when there's a pwBPD (person with BPD) in your life, the better, for sure.
You're really not alone in wondering -- is she "doing better" with someone else, and if so, why?
Briefly, while both partners may contribute to a particular relational dynamic, you can't cause disordered behaviors or a PD in another adult.
You contributed your part to the unique dynamic going on between you and her... and so did she. However, you aren't responsible for her disordered choices, traits, and outbursts. Yes, things will be different in some ways between her and another partner, just because people are inherently different... not necessarily because "he's better for her".
I can tell you this after watching my husband's kids' mom (many BPD type traits) be married to someone with many NPD traits for >10 years:
there's no necessary connection between relational
longevity and relational
health.
The fact that a relationship lasts longer, or "looks" different, or "seems stable", doesn't mean it's a healthier relationship.
There can be an unhealthy stability/duration to some disordered pairings (like BPD/NPD), but the fact that the cycle between the kids' mom and her husband has kept going for >10 years doesn't make it amazingly healthy. It isn't, and it's pretty in-your-face dysfunctional, if you have eyes to see it. However, social media allows an artful curation of images/posts that can create the illusion of "amazing, insightful, deep, profound love, incredible commitment, emotional health" etc. I guarantee you it isn't real.
We can definitely keep chatting about that here, as "is her new relationship healthier than the one she had with me" is a big, recurring theme for so many.
In the meantime, I'm curious if your therapist has given you any feedback on your question here:
I don't get it. Did I cause the issues in the relationship? ... It makes me feel terrible. Like I'm deeply flawed and unaware. What's going on?
What did s/he reflect to you about whether you caused the issues, and whether you're unaware?
Often, if we're even open to the fact that we might have been unaware, that might indicate we're less unaware than we think. If that makes sense!
Let us know your thoughts, and again, welcome;
kells76