Sorry it has been so difficult for you. I think maaaaany of us here can relate to your experience. Are you seeing a therapist? I helps SO much.
This post from another thread helped me understand some my partner's marriage/engagement related behavior:
When a BPD knows they have you, their devaluation tendencies increase tremendously. For some BPD the "I have you" point is reached when the non shows he is in love, for others it is when they move in together, still for others it is when they are married. For them, marriage is not about love like it should be for healthy individuals, because they cannot feel love (even though you have convinced yourself they do but their actions always show otherwise).