Hi Eco,
I think you handled the situation with the bear beautifully. My guess is that your ex has feelings of jealously over this bear as silly as it sounds... .In my experience pwBPD's do often get this way even over inanimate objects. PwBPD are just no good at sharing love and affection with anyone.
My thoughts are that by having the bear you made together in your exs home she would feel like it was taking love away from her and your d would pay it way too much attention... . as silly as this sounds. Also I can see abandonment issues here. The more your d loves that bear the more unloved and abandoned your ex feels. I wouldnt try to encourage your d to take the bear home to exs home anymore as it may just "mysteriously" disappear or come back in tatters.
Another idea could be that you take loads of photos of your d while she is with you but making sure you include bear and put them in a scrapbook for her. I used to do this with my children and their favourite toys and we used to have fun looking back at them and it brought back many good memories from their childhood.
