Help!!

My fiancé and I have been together for 9 years. My daughter was 9 months old when we got together. He was so amazing and took on the roll as a father immediately. We talked and I told him that she was all he knew as a father and of he didn't want that then we needed to not go any further with our relationship. He told me he loved every minute of it and eventually wanted to start a family and have more children. Fast forward to now my daughter is 9 years old and we have a 4 year old together. I feel like he resents my oldest daughter because she isn't biologically his. I feel like he's always picking on her and expecting so much more out of her than she should have to do. As I said, she is 9 years old! I feel like I'm stuck between a rock and a hard spot. I love him with my whole heart and we have an amazing relationship. But just the way he acts sometimes, I feel like my oldest daughter and I are rather burdens to him. I have no one to talk to, that's why I am posting here to hopefully find someone that has gone thru the same type of situation and what their outcome eventually ended up. I just don't know what to do or how to feel about everything. He treats our 4 year old like a queen and to be quite honest, she's so naughty. And I also feel like he is making me resent our youngest daughter because he is always comparing the two. Idk what I need, just honestly someone to talk to. 😥