A few months ago my girlfriend 'accidentally' told me about her ex-boyfriend being over 8 inches. We have never done anything and she hasn't seen mine yet and she asked me my size and I told her 7 (which she didn't even consider big) and a few weeks later I told her the truth that I was just under 6.
I told her I was 7 because I felt pathetic after finding out about her ex, and I'm still kind of mad about her telling me about him, and worried about doing stuff with her because I feel that as soon as she sees mine she'll automatically think "wow, my ex's was so much bigger".
She's told me that she wouldn't compare and that I shouldn't be worried and size doesn't matter to her, but I can't help but thinking that.
Comparison is an intelligence trait. We contrast everyone we know based on how they measure against other people. She will without a doubt be aware of the differences between your penis and her Ex's and any other penis she's seen in her life. And without a doubt she will care less about this than you do.
Think about how your girlfriend compares to every other girl you've been in love with. Then think about how few of the things you're comparing have to do with sex versus her manners or kindness or intelligence. She's not with you because of the dimensions of a piece of your anatomy she hasn't seen yet and once she sees it, it still won't be a deciding factor in what she thinks about you.
Also never be mad at someone for giving you information. She has told you this doesn't matter to her. Any problem you have with her Ex's anaotmy is entirely on you for being insecure about your abilities.