My grandfather was saying bad things about my girlfriend and I'm not going to stand up for her.
Is it because he's very old and you're worried about his health if you have a confrontation?
They are rational reasons behind not standing up to him, but ultimately I'm weak.
If he is attacking her verbally or physically you must stand up for her. Don't allow your grandfather to bully your girlfriend.
I'm going to stand up to him now.
That's very sweet of you, to finally stand up for your girlfriend. I am sure she'll be happy that you are going to protect her from an attack, even if it is from an older person, it must suck to hear bad things about you.