Nat:
We b-moms sure do feel that way at times!!!! There are times where I have gone into pure selfish mode, and then had to step back and go "silly, this is what you CHOSE to happen for a good reason! This is what you wanted!" And in my situation, it really has worked out beautifully in regards to what a wonderful life my DD has had, and how her a-parents are truly wonderful people. In reality, there is little reason for me not to feel blessed by it all and I know that but of course, I'll whine anyway, LOL!!!
I don't use qualifiers outside of here. If I am talking about my DD to my friends, I call her "DD's mom", because that's what she is. I wrote my DD a letter last year, and in that letter and in her journal, I always call a-mom "your mom". I don't know, I know I'm her mom "too", but I just don't feel right having her call me Mom. It's not that I don't feel that I deserve it or I'm worthy of it, I certainly have motherly feelings for her, wanting her to do well and doing whatever I could ever do to keep her safe and protected, I'd give my life for her in a heartbeat. And I've acted that way since the day I knew she existed. But on the flip side I didn't parent her and I think the same way, that there is only one MOM. It's not me, and I'm OK with that because I made that choice. I know that's not a popular birthparent standpoint, but that's how I feel.
Now if down the line she wants to call me Mom, well, I'm not going to tell her no of that's what makes her truly comfortable, but that would be a decision she would have to come to as an adult on her own, KWIM? But I would encourage her to be respectful of her mother's feelings about that.