Advice for a new stepmom

My boyfriend and I have been together for two years. He has a 3 yr old daughter from a previous marriage. It was so easy to love her in the beginning because she was still a baby when I met her. Now she’s going through a really jealous phase. I can’t get near my boyfriend without her throwing herself on the ground and screaming. I’m trying my best. Her new favorite thing to tell me is I only love daddy. It actually hurts my feelings. She is also kinda hard to deal with at time so I can’t help but get frustrated with her, I won’t yell at her but I am firm on her. She loves to yell your mean, I don’t like you. My boyfriend and I are in a serious relationship and talking about marriage but he thinks I need to try harder to have a better relationship with his daughter. I don’t know how else to win her over. I feel like my boyfriend thinks I’m not even trying. Please, any advice is welcome. I’m so new to this kind of situation.