My Buttercup, I was her Jenn...
I am a forty year old woman that has been through much in the past 4, well I guess 8 years. I am a BD and marketing professional for the past 15 years, though currently unemployed. I only mention this as my ex significant other and I ran his aerospace company for five years. In that time I also raised his beautiful little girl from 18 months until she was 6 1/2. My ex fired me when he got his new girlfriend pregnant (she wasn't really new). I left him due to physical and verbal abuse that his daughter started to see. So in one phone call lost my baby girl, I called her my buttercup and my job. I have no biological children of my own, so she was my only. This loss has affected everything in my life. I am not sure where else to turn, as friends and family do not understand the bond I had with her. I am in so much pain, but the hardest part is people think I am crazy and to just let it go....but she hasn't died. She is alive and I raised her in the most critical stage of life, where children learn and bond. I feel so much pain that she will go through abandonment issues later, as my mom left me at 6. I am not sure where to turn, as it, this hurt will not go away. So perhaps finding someone with empathy for my loss would help...She once asked me, "Does everyone have a Jenn?" The sweetest words I ever heard. 
I think that I'm about to go
I think that I'm about to go through this with my boyfriend and his son. We haven't been together long, but his son and I bonded the moment I met him and I really love that little guy. The last time I saw him, he asked me if I could be his real mom and said "I want you to be my real mom because you're the one that cares about me". I always say "are you ready spaghetti?" and he would respond "you got it meatball!" so I started calling him Spaghetti and he started calling me Meatball.
Honestly, I think that he's the only thing that's keeping me and his father together, I don't want to hurt my boyfriend's son by breaking up with my boyfriend, but I don't feel like my boyfriend really wants to be in a relationship right now, even though he swears he does, his actions say otherwise. So while I can't even compare to the pain you're feeling, I definitely feel like I would (or am about to) lose something (and someone) very special and important to me, along with my boyfriend who I love very much.
I'm so sorry that you're going through this. That little girl was lucky to have you in her life and she'll always remember you. My mother wasn't around, but those mother figures that were there for me are who I consider my mothers, even though they are no longer in my life. I will never forget them.
I hope things get better for you. Give yourself time to heal, it won't be easy, but things will get better. And you don't have to forget about your step daughter, you can always remember her. Send me a message and I'll give you my email if you want to talk/vent beyond this forum. I can't say I know what you're going through, but I definitely feel the same about my Spaghetti that you feel for your Buttercup.