How about if I start refusing to help my girlfriend's mother?
Why don't you want to help your girlfriend's mother? I find this hard to believe and would hope this was not real.
Her mom has some serious mental problems and can't control her emotions sometimes.
Helping your partner's parents is nice. You should tell set some boundaries with your partner's family around what you're willing to do. You can still be supportive, but only in your own capacity.
My girlfriend is a big girl now, she needs to learn to deal with her mom.
Your partner has a lot of things on her mind when it comes to her mom. Helping her out with that is being supportive and something you should consider. It doesn't take much time out of your day and you need to care about your partner's mental health.