It must have come as such a shock to you to have discovered your dad's stash of drugs, especially considering the things he said to your mum. However, I think I can see why he is so angry with your mum, although this doesn't excuse his behaviour at all. To me, it seems like he's angry at himself for doing the drugs, and taking his anger out on your mum is easier than him dealing with his own problems. Your parents both have some serious issues here, that much is clear, maybe your dad more so than your mum, considering she is open about her addiction, whereas he feels the need to hide behind a mask. It's really great you're going to a counsellor, but I think you're being slightly impatient. Therapy isn't a "quick fix" process. It's often very difficult for people to open up about their problems, and on top of that, therapy isn't the best thing for everybody. A lot of people don't like being questioned by a total stranger. Is anyone else aware of these problems? A family member, or even a teacher? I think it's important that another adult is aware of what's going on here, because it's clear you're going through a tough time, and you obviously need some support.
Please, try not to lose faith in your counsellor. She's trying her best, but often, people can't be helped until they want to be. You can't force people to get help and treatment if they don't think they have problems, after all. I know you're probably really frustrated here, but you need to try and stay strong - your parents are obviously going through a great deal, and it's really unfortunate that it's impacting on you so much.
Good Luck (: