What does it mean when a girl says “don’t forget me.?

No idea. Ive never said it

She's either keeping you as a backup plan (in the case a relationship was possible) or she knows that she isn't worth remembering.

She wants to stay special in your mind.

Always best to ask the person who said it.

But failing that, I'd guess it's a nasty way of saying goodbye.

I think it means "pour me another glass of wine and rub my feet" if I'm not mistaken.

It means as soon as she knows she's in your will she's gonna stick a knife in ya!

Probably means she messing around

god help you if you do

She wants you to continue to pine for her.

Maybe she thinks you will leave her..