This is a process and it's not going to be easy to rewire yourself overnight. You were given a test with this situation and it's ok to fail at first...it's tough! It's important that you realized you went against what you and your T had planned and that you are aware it doesn't feel right. That is a huge first step!
I'm sure your T won't be harsh on you about this. Just be honest and be sure to explain how all of this made feel, and why you felt the need to still accommodate your father. Your T could help you figure out why it was so hard to tell your father how you felt and maybe he can help you with dialogue.
Has he asked you exactly what you would like to say to your father? It's one thing to know there is plenty to say, but another to organize the thoughts to effectively and calmly communicate them.

I would be surprise if your T is upset with you over this instead of helping you work through the "why, what, and how" of the situation.