I have a similar experience to this very recently. My DH has told me that he is a gambling addict, has been since he was a teenager and therefore all the while I have been with him.
I was proud of him for opening up to me and admitting it, not to me, but to himself, as a PP that is a massive step and one you should applaud him for.
I can’t even tell you how much debt he’s got himself into but for me it isn’t about the money, it was about the lies and the deceit. I didn’t yell or scream, we didn’t argue, I just sat and listened to him. He too said he wanted me to support him in trying to stop but it soon became clear that he needed more so now he attends GA and it has massively helped. He has opened up like he never has before, he is also suffering mentally because he used gambling as a distraction but now he has to face his demons head on.
He too had mood swings, often telling me there was nothing wrong and me walking on eggshells not wanting to upset him but things have been so much better since he admitted it.
My advice to you would be to look after yourself, be there for your DP when he needs you, but you need to be your priority. Don’t be hard on him, at least for the time being while he is trying to do the right thing. It would have been hard for him to admit this to you, and he will be beating himself up enough without you doing it for him, that doesn’t mean you have to pander to him but do try and understand his position and voice your concerns without it turning into an argument - I don’t find arguments productive.
He needs to explore why he has become dependent on them, that’s the only way he will beat the addiction.