Hi wondering, really sorry to hear this. I hope you were able to spend Christmas with loved ones instead, or doing nice things for yourself?
I have done a lot of reading around here over the last few months and being blindsided by a breakup with a "recovering" alcoholic seems to be a fairly common experience, sadly. It seems to be kind of their MO. Sometimes it might be that they are relapsing, but more fundamentally it seems to me that they need a lot of space, do not deal with the stresses of life very well, and cannot handle being responsible to another person.
It's difficult because at the start they seem so self-aware and that is part of the appeal - they have identified and are working on their issues with alcohol, they quote the Big Book / self-help principles, they're committed to making the relationship work, they're refreshingly honest and open about their feelings and their plans for the future with you! Ultimately though this is all talk, not action - I think many of them cannot live up to their (freely given) promises, as much as they would like to, and cannot handle the responsibilities of a relationship. There is much more work to do than simply refraining from drinking and attending a few meetings here and there. However, it is much easier to blame things on the other person or the relationship not being right than it is to face these issues.
Of course, everyone has the right to decide to end a relationship that they no longer wish to be in and it does not have to be a mutual decision, however it's not normal adult behaviour to end things with no warning or discussion beforehand. It leaves a person reeling and makes it much more difficult to move on, because there is a whole heap of processing to do first, and the other person is no longer available to discuss any of it (while still within the relationship)!
I'm glad you're planning to attend Al Anon meetings and that you are being strict with no contact. In terms of other things that you can do to find closure, I'm sure you've heard this but closure has to come from you and it will never come from him. Trying to obtain an answer or seek closure from him now that you are outside the relationship will get you nowhere, especially because (as you said) he might not even be able to admit to himself what is really going on. You get to decide why you think the relationship ended, and what you can learn from it.
Personally I found a (short) Ted Talk by Guy Winch really helpful, titled "How to Fix a Broken Heart" (sorry my post count is too low to provide a link to it!). He likens getting over heartbreak to addiction, and has some good recommendations.
I hope you keep posting here and reading around, I think you will find many stories much like your own. I'm so sorry you're dealing with this.