I know EXACTLY how you feel. I have been going through the same thing for awhile now. I actually posted something like this awhile back & I think the advice that really brought it home was that I am my childs only advocate. It is my responsibilty to make sure she is safe & happy. My 11 y.o. acts the same way to my 9 y.o. My 9 y.o. is also very sensitive & I think my 11 y.o. plays off of that. (Kinda like a bully) While I love my 11 y.o., it's not acceptable behavior & I WILL NOT put up with it anymore. I think once I finally stepped up & forced her to finish out punishments (which is so hard to do with 4 kids especially when you're so busy you can't even remember punishing anyone :) ), she is slowly not doing her bad behavior as much.
Also, it kinda sounds like your ex is manipulating you & your oldest son. He should be trying to help the situation, not pushing a bigger riff between you & your son. I think if you have the room, he should get his own room. He's twice the age of your other son & they just don't share the same interests. Maybe things might change a little if he had some of his own space.
I'm so sorry you feel as if you're heading for a breakdown. I guess I've been there before, & it's no fun. If you feel as though you need to talk to someone, you're more than welcome to p.m. me. I have 4 kiddos too-& life can be insane, to put it mildly.