I read about Borderline Personality Disorder. Although I'm not 100% sure and it would be up to a Professional to diagnose, my ex appeared to have displayed some of the signs. For example, the intensity of the relationship and how he seemed to have idealized me. How after the 2nd date he really liked me and changed his mind about wanting to take it slowly. After breaking up with me, his behaviour changed to how he used to be 15 years ago and before he was married (according to his cousin) and his cousin said my ex turned very selfish and back to womanising. The day after the break up, he re-activated his Facebook account and spent hours on there, which I believe was to contact his female Facebook friends. Some of them were ex girlfriends from before he got married. I suspect he eventually hooked up with one of them who has recently been separated from her husband. I believe I saw them by chance. He was driving a convertible and she was the passenger. My ex did not have a convertible, but on the night he broke up with me, he told me he wanted to buy an Audi TT and an expensive watch. He was in debt and didn't have a job. He declined to continue with a training course that his cousin paid for to help him get a well paid job. Instead, he took an unfulfilling job. I didn't experience the devaluation phase (not to my knowledge), as we didn't live together and I hadn't seen him for 1 and half months. He might have done it in his mind.
Having said that, all of the above might just be abandonment issues and regression from his depression. Personality Disorders are, I understand, very difficult to diagnose. What I know for sure is that my ex is sick but refuses to seek help and treatment.