K.,
Looks like you've gotten some good help...I just wanted to offer some sympathy! I get drawn into those yelling matches with my 5 year old daughter! Five, not 12, so isn't that pathetic?! And my two year old is picking up on it, some I'm having to change my own behavior FAST before it becomes her habit, too.
I think every situation is unique, but here's what I've put together in my own: sometimes my yelling is because I already feel out of control or inadequate, or just not heard on ANY level in ANY area of my life. I also had to realize that alot of times, I just don't want to listen to my five year old's rambling way of asking or telling me something. I don't have enough patience! I'm making an effort to stay (pretend) calm, slow down, and listen, and to quietly correct her when she is loud and talking back, etc, and I'm trying to really focus on praising the good things, instructing about the "bad" things without getting personal (as in, "Why didn't you pick up your toys!")
We have also set up a few "natural" consequences to happen when she doesn't do what she's supposed to, and that helps it become just a natural thing and not a punishment that I'm coming up with in the heat of the moment.
The advice about remembering puberty is good, because I know that escalates emotions. I have much admration for you - it is not easy being a married SAHM, so I can't imagine how much more difficult to do it as a working mom with an ex who isn't as reliable and supportive as possible. Good for you to go parttime! That takes so much courage and committment. I wish you the best in that, and I'll say a prayer that finances will not be too strained for you. God bless you! Hang in there.
S.