Soon to be step mother

Ok for all the women out there, please help me!!!! I have no where to turn too.....I dont know who to talk to, so I will talk to strangers. Please help!
Here I go...

I am 18 years old soon to be 19, My boyfriend and Myself are gorwing to be a little more serious about our relationship. I do infact lover him very much and I would marry him! But I would not only be marrying him, but his daughter from another relationship. He is only 22, I thought I was ok with this, but as we spend more time with her I am confronting her mother more often. Please dont get me wrong, I love this little girl with all my heart and it breaks my heart everytime we send her back. I would only be so lucky to have this angel in my life. Yet his ex is making me sick, I get physically ill everytime I encounter her. I cry when we drop the little one off because I am so jealous inside my body I want to hop out of the car and slam her down. I hate her very much, I dont know why, I just do. I get mad that they talk on the phone, on the internet or face to face. I despise the fact they were together so much I question this relationship. I always dreamed when I got married I would be the bride, not the bride and a stepmother. There is so much to think about. I want to have a child, what kind of stress will this bring into my relationship with my husband and stepchild. I never want either of them (my biological child and non-biological) to feel as if i dont love them equal. I would very much, and what if she grows to have ill feelings toward me like I'm "the other women" I dont know that she would understand what really happend when she was only an infant, that her real parents are not married but her father married to "another women" my heart is so broken and tore up right now, I dont know what to do....please help.....there is so much to this story.......