Feel like my fiance's 2 year old doesn't want me anymore...

So I've known my now fiance 6 years. Dating since last winter, engaged since the late summer and were very happy until the last month. She has a 2 year old, 3 next summer, who's father will not now or ever have anything to do with him. So essentially aside from my fiance's dad, I am the primary father figure in his life. I love the kid, I really really do even if he frustrates me sometimes, but lately I am at the end of my rope. I see him and her 2-3 times a week, always the 3 of us together, usually staying several nights over. Our usual routine all year was he would rush to me when I got over and give me a hug, sometimes even yell "Dad!" He slept great, he even left me and my fiance sit and cuddle while he played or used ABCMouse. Life was bliss. We felt like a real family.Then he got sick, like 3 illnesses in a row sick, and all he wanted was mom. If I touched him or even sat near him, he cried, threw a fit, and would SCREAM until my fiance came over. The only time he wanted me was bedtime because I am warm and he had the chills at night, so he would cuddle up to me. He got over his illnesses, as toddlers do, but not much has changed. He doesn't scream and cry when I sit near him, and he's fine (usually) when my fiance is out of the room. He'll sit and play, hug me, giggle, everything. But if she is in the room I do not exist. If I try to have him sit with me, tantrum. If I tell him no to anything, tantrum. If I block his way to her for even a minute or she doesn't give him immediate attention, tantrum. If we sit and cuddle and he realizes he's not included, tantrum or whining until she cuddles him, usually leaving me alone on the opposite end of the couch.Just most recently I told him no to getting into her makeup and carried him over to the couch. The second his butt hit the couch, tears, screaming, biting his finger, and flailing. My fiance rushed over to him and he stopped and she calmed him down. Not a minute later we were sitting talking and he looked at her, looked at me, I smiled, and he threw himself on the floor and started screaming again. She says it's a phase, she's his mom, I know she knows, but it frustrates me so much, and I feel like I lost whatever bond I had developed with him no matter how hard I try. I don't know what I am doing wrong and it makes me feel I am not ready to be a parent. Am I in the wrong? Am I being immature? I just need advice. via /r/Parenting http://ift.tt/2zUbuep