Well, eventually he will have to take responsibility or he will keep losing priveleges. Maybe try setting a short-term goal that is realistic for him, fairly easy to achieve, so he can see a reward for good behavior. Then slowly increase the difficulty.

He's saying those things because he's mad and wants to hurt you and/or get you to back down. I'm pretty sure he knows it's his own fault but he's mad. You have to stay strong and decide you're not going to let his ranting get to you.

As for me and my parenting style, I don't stand for fits. We all have a right to be sad/mad/frustrated/angry but we don't have a right to take those feelings out on anyone else. If my son wants to mouth off like that, yell, scream, whatever, he goes to his room until he's done and calmed down. If going to his room is not an option, then we start adding days to his grounding and as soon as we get home they go on the calendar. He does not get away with disrespecting me or anyone else.

Listen, I know it's not easy. You have the hardest job on earth and you're doing the best you can with what you know. These little stinkers don't come with instruction manuals. I'm just giving you bits of info from my own experience and maybe there will be something you can use. If not, at least know there's one more person supporting you.