Can he just not move past the mistake I made? Is it hard for people with BPD to trust again?
Hi and welcome. Sadly, it
is hard for BPD's to trust, even if you've never given them cause to doubt you, because insecurity plays a big part in their condition. They fear being abandoned and even the slightest and most innocent thing you do - like simply turning up late - can make them think you don't care any more and are preparing to ditch them. Their thinking can be so disordered and they see a breakup threat in everything.
Knowing that you did sleep with someone else while with him will most certainly have triggered him, hence his unwillingness to commit and officially call you a serious couple. Plus BPD's tend to bring up things from the past and use it against you whenever they have an episode - - they seem to have a remarkable memory when it comes to remembering past 'insults'.
Regarding him enjoying being chased, I can certainly relate to that as I chased after my exBPD whenever she broke up with me and I'm certain she enjoyed it and the power she had. Though a lot of BPD's actions can be put down to their illness, I still believe they know exactly what they're doing a lot of the time.
All you can do is to give him no further cause to doubt you and try to build up his trust again - which is still not a guarantee as he can see things which aren't there and interpret your every innocent action as something bad. His BPD would have shown up whether you did anything to him or not, it's just the nature of the condition.
Best wishes
