My relationship to an abusive "recovering" (he just didn't drink anymore but didn't really work on himself either) alcoholic ended just a few months ago. And even though he was no longer drinking, his behaviour seemed so irrational, over the top, it never made sense. In the beginning he came up with excuses, then he stopped giving any explanations and just blamed me for all his actions or denied them.
I also felt like I needed to know why he behaved the way he did in order to close this chapter of my life. I tried to find answers with my therapist who suspected him to have some borderline tendencies. It never really added up though. On this forum the book "Why does he do that?" by Lundy Bancroft was recommended to me. It tries to give answers to the many why's and to explain the men's thinking behind their actions. For me it did exactly that and I found it helpful in many other ways. I always thought of my ex as an unique unsolvable puzzle. After having read that book that changed completely. His behaviours are neither as unique, nor as random nor as inexplicable as I thought.
I don't know much about your relationship but if your ex was abusive and you wonder why he treated you that way, maybe the book can be helpful to you too.