Hi there
It sounds like you miss your dad very much and life has been difficult. You say he did those ' bad things because of alcohol’. Your dad was hitting you and your mum over a long period of time and threatening your safety, which is not good.
It seems that your mum wants you both to be safe and this wouldn’t be possible if there was contact with your dad. It’s ok to care, miss and think about your dad. I'm sure he knows this too.
It sounds as though you and your mum and her new boyfriend get on well. You have done the right thing asking him not to speak badly about your dad.
Maybe if you find it difficult to talk to your mum about all this and you want to talk ChildLine make contact on either 0800 1111 or have a 1-2-1 chat. ChildLine is always there for you.
Take care
Sam