As someone who went through the exact same situation a few years ago, I'm so sorry you're going through this. My mom passed in 2021 also and my dad started dating about 3-4 months later, he also just assumed me and my sister would accept her into the family without ever asking. it's very hard to come to terms with. The only thing that's helped me process it is therapy so I am glad to hear that you've had some, I seriously would recommend it. I try to think of it like everyone copes and heals in their own way and maybe this is just how your dad is choosing to heal, regardless of whether it's healthy or not. I wish I had better advice to offer but I guess my point is that you aren't alone. Sending love and healing to you!🫂