I don't think this guy can do right by a relationship with you or with anyone else, until he more fully disengages from this past relationship. You're probably not going to get him to see that right now. So you have to enforce some boundary for yourself. That could mean telling him, "I won't be able to stay visiting you, if our being together gets interrupted by this gal appearing at the door to get some of your attention." Then you'll have to follow through by actually getting up and leaving, if he ends up at the doorway yakking with this attention-hound. That's what she sounds like to me. I suspect she is not willing to fully let go of what she once had with this man. It's no coincidence that she keeps appearing when you are with him. She is there to deliberately intrude on your time with him . . . IMHO. Tell him you do not want to have any conversations with this person. No reason why you should. Tell him to please not open the door to her, while you are there. She should not be just showing up. If she needs to see him, she should text him. Then he can tell her when it is convenient for him to see her. That should not be a frequent occurrence.

If she has no concept of "boundaries," then it's high time people started teaching her by setting and enforcing some. It doesn't have to be done in a mean, angry way . . . but it should be done. You can't mandate what boundaries your boyfriend should have. He's in charge of that. But you have the right to say what you are not okay about. Tell him that you want zero contact with his ex-girlfriends. If he has to arrange that, while you are there, it may inspire him to do more to cut her off, even when you're not there. An island can be a small world, but I'm sure not everyone on that island runs in to everyone else, as often as she manages to run in to him. She probably uses her personality disorder as an excuse for behaving like she doesn't understand normal social boundaries. Too bad. Time for her to knock it off.

Your reaction is not immature. I will say that he can't really apologize for her. But, he also can't have his cake and eat it too. Tell him that. Say, "I don't date guys who are still semi-involved with girlfriends from the past." That's a decision you can make for you. You may risk losing him, but, if you take that risk, he'll either drop you or show more respect for you and your legitimate objections. If you won't take that risk, get ready for more disrespect. We teach people how to treat us.