An antidepressant will make you feel tons better. Even if you don't feel "sad" most of the time, it will fix your irritability and moodiness, even on the other days.
Sometimes you need to try a few antidepressants before you find the right one for you. I'd start with Prozac (because it is on the $4 list and is approved for PMDD), and if that didn't work, try Celexa or Zoloft.
And don't feel like it is some big deal to start on an antidepressant. It won't make you a zombie or anything. It will make you a better mom because you will be able to handle things better.