I’m not sure what BDP stands for but I’m assuming it’s borderline personality (?). I’m not a fan of labels in daily life. They’re useful in a treatment context, but in daily life can easily become a crutch and an excuse for bad behavior. I'm a strong believer of people rising above their limitations rather than spiraling down around them.
You didn’t clearly say why you and your bf were apart for a month, but I gather it’s got something to do with the BDP. I think the bigger problem you have is with this guy’s level of commitment to you. Forget the other woman. She’s a minor detail, like a subplot the screenwriter will ignore when s/he adapts the novel into a movie. If I were you I’d sit down with the guy and have a very important conversation of what it means to be in a relationship with a borderline (if that’s what the BDP was). It’s not a walk-in the park as I’m sure you know and whole books have been written about it. If the guy is not fully up for it, let him go. You’ll be saving yourself a lot of heartache down the road. If he is, he’ll have to show the level of maturity it takes, and that excludes sharing personal details with strangers or flirting for the sake of drama and excitement.
And no, I don’t think you’re a sociopath. You said you have empathy. Sociopaths don’t. Usually two siblings of a traumatic childhood tend to go to polar opposites. So even if you understand your sister, that doesn’t mean you’re like her.
Like the previous comment suggested, focus on being productive and moving forward. Don’t get stuck on tiny ripples but swim past them. They may seem big now, but some day when you look back you’ll realize they weren’t.