Step parenting: the fundamental flaw
Step parenting is going into battle bareback with wooden weapons against an enemy with nuclear missiles.
It's like going to a party where you know nobody except for the host, who for obvious reasons can not attend to you all night and all you can do is look on from a distance and awkwardly smile at the closely knit crowd of people, desperately trying to engage with anyone that your erratic eyes make contact with as your inner voice begs and pleads for someone to acknowledge you and end your social exclusion.
The biological parent has been with the child since day one, learning, moulding, adapting, teaching and instilling. You weren't apart of that process, the child does not share your morals or values. The child does not know what you find acceptable and not acceptable, what you will tolerate and what you won't tolerate. You don't have that history, that's what i think makes being a step parent so hard.