Hi and welcome to the group
we're glad you found us and felt ready to share your story.
What you went through sounds so familiar here on the Detaching board: it can seem like the pwBPD (person with BPD) is happily moving on with life, posting amazing pictures on social media, "never been happier", etc, all right after ending a serious long-term relationship. It's no wonder you feel like you're in shock; the break up came out of nowhere (for you) and in the middle of big life plans like building the house and possibly getting engaged.
In terms of whether he'll regret his decision or not, it's hard to say. I wouldn't depend on what social media portrays to get an accurate picture of how he feels. Social media allows people to curate an image of themself and their life, so you're only going to see whatever he decides to post. Right now, he wants to portray an image of "happy family man", and there isn't really any conclusion we can draw from that image about what he may or may not regret. I think all we can conclude is that -- that's what he wants people to see of him right now.
I wonder if a bigger question for you would be: are you still a worthwhile person, even if he didn't regret breaking up? I think we all know intellectually that the answer is Yes, we have value regardless of what the pwBPD thinks or feels.
But it's more a question at a feelings level. How would you feel about yourself, no matter how he felt?
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In terms of wondering about if he'll reach out again, one approach is to just assume that he will sometime. It's not uncommon -- though each pwBPD is different. Many pwBPD fear being alone or abandoned, and so if there are difficulties in a new relationship, the pwBPD may reach out to a "known entity" for support and to not be alone.
Again, though, a big question for you might be: no matter what he does or doesn't do, what do you want? Are you done with the relationship?
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pwBPD struggle to see a balanced picture of partners. They may have "black and white" thinking (all or nothing thinking, either-or thinking) where if you're good, you're great, but if you have any flaws, you're awful. So it can be true that overall you were a great partner, yet due to your ex's disordered thinking, any flaw you had or mistake you made took over his viewpoint and he couldn't see the full picture, only the negatives. A partner could be a perfect angel and still a pwBPD could find a "reason" to break up. That's not to say that the non does not play a part, more to say: confusion about "how could he break up with me, I was actually a pretty good partner" makes a lot of sense in your situation. One of the tragedies of BPD is how it seems to blind sufferers to the love and care of people around them.
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You mention working on yourself; how is that going? Are you seeing a counselor or therapist, and are you able to work on the betrayal you experienced?
