I'm going to back out of a plan with my girlfriend
Why are you backing out of the plan?
I'm thinking of backing out because my friends invited me out.
It's rude to cancel plans that you have made with your girlfriend, because she was really looking forward to spending time with you.  There will be other chances to see your friends, so you shouldn't abandon all of the planning that you and your girlfriend have already done.
I'm not responsible for her feelings. She can get over it.
So you don't care how your girlfriend feels? That is incredibly insensitive. Why are you with her?