Hey elessar,
To answer your question from my experience commitment is something our BPDexs dont have the emotional development to provide. For example a young child isnt ready to be married let alone share and intimate relationship. Since our exs are essentially stuck at that emotional level commitment isnt in there capacity to provide i dont believe other than moment to moment. Alot of it has to do with fantasizing commitment then actually being able to fallibly hold up the idea to match their actions. For example my ex got me sent to jail on some trumped up charges after her rage/faux breakup and i was out a few weeks later. We reconciled, went to a music festival, and had an amazing time. Well during the end succeeding into the weeks to come she started wanting me to call her my fiance and was bringing up marriage. Then the idealization period started to wear off and so did the talk of being my "fiance". My point being i think they get caught up in their BPD reality and let their imaginations take control of reality for points but then it starts to wear off as they realize the emotional attachment that comes with such a commitment. This triggers abandonment fears so they take the relationship back to a more comfortable place or push us away.
I am just wondering what made her "commit" to him. If she was interested in him, why pull me in like that during the previous few months, or why make a new dating profile.
I think this has more to do about triangulation due to the commitment. My thoughts are she was feeling stress in the relationship(the thought of marriage probably being the trigger) with the new guy So she needed to bring you into the mix to relieve some of the tension/pressure with the new guy. When this didnt fulfill her needs she chose to look elsewhere for attention/validation/triangulation(hence setting up the dating profile after contact lessening with you.) I can only assume her efforts came up fruitless on the dating websites so she went back to the new guy and agreed as its security like because in a sense im sure hes her current insurance plan. To add onto this my ex would tell me specifically about one ex(her longest relationship next to me) that she stayed with because she felt secure and safe but didnt really like him but she knew he wouldnt leave her thus her abandonment fears were satiated for alot longer.(they were together almost 2 years like us) So this could be very well the explanation for these behaviors.
So two weeks after turning 30 she picked the first guy who was available because she knew she couldn't make that commitment to me.
Its nothing personal my friend so i hope you dont take this to heart. You did all that you could and i bet went above and beyond like most of us did to make these relationships work. I know it must hurt trust me i hope i never hear of the day mine is getting married. But at the same time think of this mans future it will not be anything different than what you experienced in the relationship... more of the same except theyre legally bound so its just going to get that much more complicated. I wouldn't wish that on my worst enemy...