I’m in no position to offer advice as a Mum as my twins are only 19 months old but just wanted to say that I don’t think OP is being horrible or that the issue is necessarily with her or her emotional state, but I do think it would be worth exploring that avenue though to be on the safe side.
I do have my own childhood as reference. I was raised alongside my DB (2 years older than me) and Dsis (4 years younger) by my Grandparents. My Brother was prone to extreme temper tantrums almost daily and was wildly difficult from around the age of 6/7 until his mid teens. We now know he is on the spectrum but this wasn’t something my GP’s were even aware of as a concept so just treated him as they would any other kid.
I’m not saying your DS is on the spectrum but I know that my Gran in particular was so worn out by him and she isn’t exactly the most emotionally available woman, but would be reduced to tears.
As an adult I can see with hindsight what a hard time my GP’s had raising us, I wasn’t perfect by any means and Dsis was the “baby” so had a lot of moments too. What hurts is the resigned indifference that eventually developed between my Gran and all of us, she truly did give up and her eventual rhetoric was “do what you want” when she couldn’t face the tantrums anymore. She is only human after all as are you OP.
I really hope that you can start to find some solutions. Years on from my childhood my Gran told us (whilst a bit tipsy) that no matter what she said she loved and still loves all of us and she apologised for her harshness when she said things like “I wish we’d never taken you in”, “you’ve ruined my life”, “you’re trying to break me aren’t you”. She would often tell me in particular that she hated me. She admits now I was the easiest target for her frustration because I was the calmest of the three.
My brother eventually improved when it was identified that giving him a few days notice for planned outings helped. He needed strict structure to feel comfortable and still does. When he was young being told on the day that he had to do something made him panic, so he lashed out at everybody and anything.
Being the middle child I was left to my own devices a lot and essentially played peacekeeper between DB and Dsis. Dsis had no particular issues but was emulating my DB for a few years as she saw it got him what he wanted so tried it too. Like a lot of kids she grew out of it and improved her behaviour.
I’m sorry for you that you’re going through such a rough time OP and I truly hope things improve for you 