A word about power struggles, since that seems to be at play for the OP:
I notice, first of all, that actual power struggles are just way less likely to happen with my ASC kid. He's extraordinarily flexible, for any kid, ASC or not, and so we rarely get into these situations. My non-autistic child, however, can easily be drawn into them, but I don't think this has anything to do with autism. What I just know to be true about power struggles is that only I have the ability to create them - not my kids. My children are small, innocent, and simply trying to figure out how to get on in the world. I, on the other hand, have been here a lot longer. I have tons of power with my children that I always want to be careful not to wield in a harmful way. It would be so easy to do, in fact, that I get super aware of myself whenever a situation starts.
I like to think of power struggles like this - all the best *strength in non-violence* philosophies (sorry - don't know what else to call it; brain feels like mush from limited sleep) incorporate the idea that when something pushes, you release to it, move with it, but while still maintaining your own strength. So, I bring this into my parenting. No matter how I'm feeling at the moment, I remind myself that my child needs me to contain his feelings, and I immediately become empathetic. I show my empathy, whether physically or with words or both (like, "You don't want to go to sleep; you're frustrated; etc"), but I maintain what I feel is important, like that he still has to take his nap because he's exhausted, or he still has to leave the children's museum when it's time to go, or whatever.
And, I pick what really matters to stay strong on. Like so many PPs said, milk in cereal just wouldn't be one of them. My ASC kiddo eats three things, only one of them that could vaguely be considered actual food, and as painful as this has been for me as a mother who wants to nurture her child, I have never struggled with him over it. It would only make things worse. Instead, he's in feeding therapy, and I just regularly offer him food, which he always refuses.
I don't want it to sound like I don't screw up all the time, or like I don't get fed up a lot and have to apologize, or am otherwise inhuman. To me, good parenting is often more about one's philosophical intentions. Another PP here said she thought you might be approaching your situation from a place that assumes your child will respond in a *typical* way (although, I don't think your approach would work with a typical child either), and this change in thinking about the situation really needs to be the first step you take.