From my own experiences, it is very difficult to watch your father move out of your home, marry another woman and then start a new family. It is not natural. It is not fair.
I feel so sorry for my 12 year old self

I pity "her" when I think that "she" cried at night because her daddy didn't live with her, but tucked other children in bed at night. Just awful when you really think about it.
I blamed my step-mom. She didn't love me the way she loved her own children. She didn't make me feel very welcome when I was in HER and my father's home. She tried, I suppose. But I knew she really didn't care about me.
Now as an adult, I blame my father. What a weak man to move out, start a new family and think I could be a Weekend Child. What a weak man for not putting his foot down and allowing me to have a bedroom at his home. I was always reminded that it was my half-sister's room. My step mother had a room for her wrapping paper

And I didn't have my own room at my father's beautiful home.
Divorce is not good for children. Having children with somebody you don't really love and are not committed to is an epidemic that really destroys those children. And marrying and starting "new" families doesn't always help the family.
Step children almost always get slighted.