Claire, I'm sorry to hear you're going through this. I think most people here have made the same mistakes that you did early on in their situations. I know I did a lot of reasoning, promising change and even a little begging during the first few months after my W left. As Julie said, you can't change it now so try not to dwell on it.
The DB and DR books are both great. I would personally recommend the DR book if you were to pick one.
One thing you will hear a lot of people say on the forums here is to make yourself into the spouse that anyone would be crazy to leave. If you can make yourself the best version of you that you can it's possible your H will notice the changes and rethink his decisions. If he doesn't, you'll be in a great place to have a very happy future without him.
Me-40,W-37 D7, D5, S3 Separated Oct 3/2013 T 11 YRS M 7 YRS