I agree Marti
I believe it is something called malignant hope that keeps us there. As I progressed through my relationship I began to see that the ending, it was inevitable. However, I tried so hard, the more I learned about BPD the more understanding I gained of the dark lonely unhappiness inside of him that made him act the way he does didn't make me love him less or more. I was learning and trying to put into practice all the lessons I have learned from here, but to no avail. My fantasy of him being the person I thought he was at the beginning faded, slowly and painfully but I was still up for making something long term no matter how hard it would be. To me it was worth it, there is a lot to love about him outside of his BPD. Sadly this has to work two ways.
As for the red flags, its a tricky one. Red flags are for the future, I knew nothing about BPD, wouldn't have known a red flag if it slapped me in the face. I struggle now big time with the emotional aspects, am heartbroken and lonely, suffering from lack of sleep and a very stressful job and in the long cold loneliness of the night I grieve my love and my dreams not my fantasies. Have rambled a bit, hope it makes sense

Love from
Sadly x