I avoid my girlfriend when she's on her period.
Is there a reason that you refuse to be around during that time?
I don't want to deal with her attitude change.
This is wrong and selfish of you. Your girlfriend will have periods throughout your time together. You can't avoid her all the time when that happens, so try to get used to her attitude change.
She has to stop it, it's unhealthy and it's just too much. I can't handle my girfriend when she's on her period.
It sounds like she might not be the greatest person to be around when she is on her period, but she is still your girlfriend. There is no way that you will always be able to avoid her during that time so you should just try to get used to her flucuations in moods.