Hey jacques g,
Thanks for your post. I've read it a few times over now and I see no part where you should be blamed or feel guilty. For what it's worth (even though I'm a stranger) I feel sad that your therapists blame you too.
Given I'm an outsider, I can kind of imagine how you feel, but I kind of imagine how your mum feels and how your dad feels too. It sounds like they both want you in your dad's life, but you don't want to be. It makes sense that you don't want to be too - it sounds like your dad played a big part in that divorce and struggled to get his drinking under control.
You said in your post that there's some stuff your mum doesn't know, so what does she know?
I think that to me it's okay if you push your dad away, because you need to right now - it doesn't mean that you have to push him away forever or he can never be apart of your life. Maybe pushing him away is right for you, or maybe it's unpacking those feelings so you can let him in. You are really the one who knows you best.
Hope this helps :)