Wow, such a rollercoaster, I'm so sorry you have been through all of that, .
First things first, this isn't actually about you. That might be hard to accept. You're in a long term relationship with an addict, when he was sober all was well, then the money win.
Alcoholics want to drink. There are many people who do get in to recovery, even very long term recovery, he is not, to this point, one of them. As you know, it's something that has to be kept foremost in their mind, every day, people also go back to drinking, every day. He is now back in active addiction to both alcohol and gambling.
Nothing and I mean nothing can compete with that. Not you or your child or the great life you were having up until he picked up his addictions again.
Who in their right mind would gamble away a million plus dollars, leave their wife and child and haunt the casinos and bars? Well he's not in his "right mind" so you can't really make logical sense out of it.
The new girlfriend is really just a distraction probably. He can't be alone, so there is her - who likes to drink. So there is no threat - yet to that mighty addiction, when that happens, that relationship will be over too. She is a psych nurse, she knows about addictions, but, she may well be an alcoholic herself.
You are not old, or cracked or broken! You may feel like it right now, but trust me, you aren't (and please don't say that to yourself, please be kind to yourself). It is going to take time to heal from this, but it is progressive, you get a little bit better all the time. The absolute best thing you can do for yourself is to take care of yourself, eat well, sleep when you can. Try to do fun things (even if you don't feel like it), be around people you love and that love you.
You were clear that you didn't want the active addict back with you and that is what he is. Sometimes boundaries are hard to hold to, but you have and you are not wrong.