Disengaging as a step mom....?

It’s such a hard concept for me to even consider. At this point, I’ve been a full time step mom to my SD for the past five years, since she was 3 years old. My husband is her custodial parent, and she spends every other weekend with her mother. Her mom isn’t involved in much of any aspect of SD’s life, apart from the 4 nights per month she spends with her(she lives 3 hours away), despite us actively trying to involve her. This includes school events, school struggles, doctor visits, major surgeries, etc. This all falls on me. I have a son of my own, and my husband isn’t the best when it comes to planning/scheduling/remembering things, so whenever I schedule anything for one I do it for the other to save myself the extra trip. Lately, I find myself in an up hill battle of trying to get my SD to care about her education. When my husband isn’t working he helps out, but I’m really the only one that helps her with homework, makes sure she’s studying for tests, makes sure she’s reading books every night. I work with her and make sure she’s doing her work, same as I do with my son. She just has a lazy mentality when it comes to school, and she laughs about the fact that she’s failing 3rd grade and getting 50s and 60s on all of her school work. It kills me, because I feel like I’m the one that’s failing her. I put all of this time and effort into her education and her life, and she yells at me for “bullying” her by asking her to read. Tells me that she hates me and would rather live with her mom because mom doesn’t make her do school work and let’s her play on her tablet all day. I’m the one that wakes her up every morning, does her hair, drives her to school, meets with teachers when there’s a conference, sends in the snacks for class parties. The way it’s going right now, she’s going to resent me for trying to get her to care about her education. But, if I disengage and stop doing all of the things that I have done for her for the past 5 years that she sees me doing for my son, she’s going to resent me for “abandoning” her. I’m at a freaking loss.