I sympathise with you, I get it quite badly, although not like you, I just feel really sorry for myself and am convinced that my heart is breaking over any little thing that happens. Are you on the pill? I've been advised myself that going on it would help to decrease the personal low you feel beforehand.
As far as adivice for immediate affect, I know it's difficult because even though you know you only feel a certain way because of PMT, it doesn't really help because you're stull feeling it none the less. I suppose the best thing is to take timeout everytime you feel yourself getting overwrought to breath deeply and tell yourself it's all in your head! Not the easiest thing to do though.