Hi confusedinny,
Just curious if some of you have experienced that greater commitment to your partner with BPD (engagement/marriage) has helped to ease the fear of abandonment and helped to improve behaviors.
difficult to say with fear of abandonment but don't think it matters much. The fear is not rationale so formal commitments can sooth for a short period of time but are soon forgotten. We can't solve emotional problems with facts. Emotional problems need to be solved by addressing emotions.
With respect to behavior it is worth noting that formal commitment like marriage brings people together and lowers boundaries. Lower boundaries makes it easier to blame the other side for emotional problems and easier to not solve the problems on the originating side. Generally the pwBPD will struggle more in an environment with weak boundaries. Marriage and similar commitments without boundary skills won't make the situation better - more the opposite.
There may be good reasons to marry but behavior of the pwBPD is not one.