TattyAnna - well done for calling time - that takes backbone, and self-belief.
Massive sympathy to you - it sounds as though you've been carrying a heavy weight for a good two years now. The thing about addictions is that they intervene between the addict and real life, and real people. I sometimes wonder if that, in fact, is their primary function for addicts. But I'm no psychologist.
I know you say he's a good father, loving husband but ... this isn't good fathering, or good husbanding.
My guess is that, after the initial grief (which is going to be for what might have been, what you deserved, rather than what you have, right now,) it will get a lot easier. My guess is that it will feel as though a big weight is lifted from you r life.
It is awful. It's not even fair. You've clearly done you absolute best, and put so much effort into trying to keep your family together. but the thing is, it shouldn't be that much effort. Imagine if both of you had been able to put all that energy into going forwards, rather than paddling frantically to keep it afloat. It's not on that you've had to do so much paddling.
As others will say, he can be a good father as a separated father. And he can continue loving the children if you separate. And you can be a good, loving mother. You can still have a loving, happy family.
The sad truth is, until you make it really clear that this is utterly incompatible with your vision of what a family life is, and a relationship is, on it will go. You do have the right to insist this isn't good enough.
All the best to you. I hope he sorts himself out at some point in the future, and gets to the bottom of what compels him to addictive behaviour.
I hope that you find life much easier when you're not negotiating it for two of you.
good luck.