I really hope you can sit down with your girlfriend and find out what the reasons are for the on and off pattern in your relationship. Yes maintaining relationships with BPD can be difficult but it would be good if together you could find out what she is running from when she breaks off abruptly. Maybe some couple therapy or counselling may help you both to explore the relationship.

That being said it can be an excuse as the previous poster mentioned. To find the truth of what is happening communication is key. Good luck.