I have terrible PMS too. My boyfriend is also very patient and understanding but I feel awful when I do get angry with him, because usually he has done nothing wrong and its just my hormones and impatient personality getting the better of me. He never shouts at me, and half the time, I know that if he had done to me what I'd done to him, I would be in my room crying and he would be apologising to me, rather than the other way around!!!
It's important to learn to just bite your lip. I know that it is very difficult - but dont you think that it's difficult for your boyfriend to keep on and on being so understanding? You need to take some responsibility for your actions rather than just blaming it on PMS.
It is very hard, there's no denying that. But you can do it, and if its anything like our situation, then your boyfriend deserves a kiss and cuddle rather than a scream and a shout!
Good luck.
LGR xxx