Being a step-mom is so challenging

I can't say that I'm complaining but I just need to vent. I don't have any friends that I can ask for advice about this and sometimes I feel alone. I have a daughter (biological) that's 5 and then my other daughter (step-daughter) she is 3. I've been her mom now for 2 years and we have 50/50 custody. So of course her mom is in the picture. We all co-parent as best as we can. They (her and her guy) tend to be more easy going when it comes to discipline and give in and constantly use they soft "baby" voice. Well I don't use that voice and tend to only use it if the child is 1 or younger. My other half has had it super lucky because my daughter took to him right away, her words "I love my daddy joshy". But our little one likes me for maybe 2 days and then screams how she wants her real mom, and how I'm so mean, and you can forget manners half the time. I sometimes just sit there and cry and think how "how the hell can I keep doing this?" I've gone as far as question if I truly want any more kids, even though I know deep down I do. I've pulled my hair out over getting her to just accept me, to love me (I think she does), I just want her to stop viewing me as the bad guy. I really do love both of my girls, but where as one proves I know I'm doing right, the other one knocks me back, and I'm just tired... And that makes me feel even more incredibly guilty