Wow- VERY similar background to me. My now husband was also the product of a messy divorce. He was married for 5 years and he ended up finding out that his wife was cheating on him with her boss. They separated and then he found out she also had a 3 year affair with his best friend. Needless to say, he lost both a wife and best friend in that whole mess. She also had a daughter who he had been the "daddy" to for 5 years considering the biological father died when she was a baby. His ex wouldn't let him have contact with the daughter and he had such a hard time dealing with it all.
We started dating about 10 months after he became officially divorced. Let me tell you, I came in during a time of his life that was probably the hardest. After dating for 6 months, his grandma ended up passing away as well. It was a very hard time for us because I always felt like he was sad and I spent most of the time either listening to his troubles or trying to cheer him up. After it awhile, it just really took a toll on me as well.
He didn't care about his ex either- he totally just blew it off because he said it was very easy to stop caring for someone if they do something like that to you. He compeltely shut off all those emotions. The hard part for him was losing the daughter.
While he wasn't "angry" like your boyfriend is, he definitely was sad and would sometimes talk angrily about his ex. That was it. It does get easier as the time progresses. Now, after 4 years together, I sometimes forget he was married before because we never talk about it. He's moved on and he's dealt with all the pain he had to endure that first year.
I'm not sure what type of advice you were seeking, but I can only offer my story. I provided support during his rough times and I know it made our relationship stronger. If your relationship can endure some hard times, then you know it will be a successful one.