Hi Hoope. I'm guessing he has been an alcoholic for quite a while?
Depression seems to be part and parcel of alcoholism - firstly it's a horrible way to live but more importantly alcohol is a depressant. It also changes the functions/connections in the brain. The brain becomes accustomed to that huge surge of feel good signals and starts shutting down it's own production (what might bring you happiness looking at a beautiful sunrise for instance).
All that is to say, he is different now. His experience over the last several years of your marriage is not what you experienced. Alcoholism is progressive. So yeah, he probably wasn't having a great time - but that's all about him and not about you.
The new "roommate" might be a drinker or not, perhaps not and that was a deal breaker, so he hot footed it to rehab. He may or may not be sober now. He may think he has it under control.
I only bring these things up to give a little clarity. Even if he is newly sober, newly sober people are usually all over the place.
I'm sure friends and family have stated that you are actually better off without him and honestly, deep down, don't you think you are? What did he really share with you during your time together? Certainly better off without him over the last few months he stayed. It's obvious his integrity was slipping, playing both sides of the fence. I'm sure that's not the guy you want.
I think grieving and relief that it's coming to an end seem perfectly normal. It's important to focus on yourself now (as I'm sure you have read here). What would you like to do now?