I am so sorry you are dealing with this. I know that feeling of constant failure, like you can never be good enough or were the "wrong" parent for your kid.
First, you are absolutely right to worry about your own mental health/sanity. Treat yourself well, get plenty of rest and healthy food and some exercise.
Second, just a suggestion that may or may not help (and that you may have already tried): one of the things I found when my DD was acting up was that the more I expected of her, the more she lived up to the expectations. She got super whiny and demanding and was basically acting like an entitled twit around 12, and I thought, oh, damn parenting fail, I've raised one of those kids. So I just said, very matter-of-fact-ly, you are getting older, you want more freedom, and I am happy to give you that, but that also comes with more responsibility, so you can start making dinner for us once a week. Of course she whined and moaned and complained. But by about week two, I'd hear her whistling or humming while she was in the kitchen, and her overall behavior calmed down a ton. Obviously, you can't literally force a kid to do stuff like that at that age, so YMMV of course if he is just insistent on refusing.
The other thing I noticed with her was that the more she behaved like a total brat, the more she needed to hear that I adored her -- she was just too grown-up to admit it or let me say something like that to her face. She was so anxious and insecure because of the ADHD -- always forgetting something, always wanting to be the good kid but getting in trouble because she spaced or missed something -- that anything that even hinted that she was less than perfect just sent her through the roof, like she had a huge road rash on her arm and I'd just poked her there. I finally realized that she had the whole world out there picking her apart and tearing her down, and that what she needed from me was a sanctuary from all that. So I worked really, really hard to stay calm, to "catch her doing something right," and to empathize when things went wrong at school instead of offering helpful suggestions (which she read as me not trusting her to handle it herself). Damn, that was hard!!! But damned if it didn't work over time -- the calmer and more positive/supportive I got, the safer she felt, and the more she calmed down (and the easier it got for me to find things to praise -- starting with how well she was handling her emotions!). I also had to be very very careful how I said even the good things -- I had to bring up stuff like that in the car, almost in an off-hand way, or go completely over the top and make a joke out of it, so it wasn't so intense for her.
Anyway, like I said, you've probably tried all of this -- Lord knows when I was in that situation, I tried everything I could think of. Good luck, and I hope things calm down for you and those boys.
