Thanks for sharing your situation with us all. I am sure there are lots of people who can understand your thoughts and feelings around going to your Dad's.
Is it possible for you to talk with your Dad about how you are feeling? Would you feel comfortable writing out some questions you want to ask him and his girlfriend, or maybe even showing them what you have written here.
Talking about stuff might seem awkward and confronting. The conversation may not come easy, but hopefully in the long run it will clear the air.
Hopefully you can acknowledge that yes you are a child of both parents yet you are still yourself. You have your own set of values, beliefs, ideas and thoughts about life.
When you are at your Dad's place, is there anything special you could do together to help build on your relationship?
This is a safe place to share your thoughts and how you are feeling. We all have doubts, fears and concerns in life. Sharing helps to gain greater insight and understanding.
Cheers from Dools