Thank you for writing to Just Answer.
It seems your girlfriend, in the spirit of being open with each other, told you what she probably just considered an interesting fact...that she had seen an 11 inch penis.
Many women do not realize how sensitive men are about penis size. This is because the size is not significant for them, and they forget (or don't realize) that just because it isn't important to them, doesn't mean that it isn't to a man.
Obviously the 11 inches was not enough to keep her interest, as she's with you, not him. And this is what matters!
There is a really good book that explains a lot about the differences in how men and women think...I learned things from this book that were never covered in my many years of training. Even though the title is geared toward marriage, the information in the first few chapters should be required reading for everyone as they get out of high school and into real relationships.
Reading this book can help you realized that what matters to women is emotional connection and the size of genitals has no bearing on that. And reading it can make her more aware of what is important to men: feeling strong, competent, and at least somewhat in charge (protective), and how even a comment with no harm intended can be very wounding to him if it makes him feel less so.
I think you both can benefit from reading How to Improve Your Marriage Without Talking About It
Don't make yourself miserable over her comment. I can say with 99% assurance that it was not intended to hurt you...but that she saw it as an interesting fact about a freak of nature, etc.the way you might think about a ridiculously large pair of breasts.--- Interesting, but not what a relationship is made of.
To stop thinking about this, whenever the thought comes into your mind, say to yourself "stop!!" and purposely change the subject. This is a technique of cognitive behavioral therapy called "thought stopping." This is to break the 'habit' of letting this thought occupy your mind. Then think of a mental image of a woman with breasts so large they are freakish--this should bring a smile to your face and break the painful habit of the first thought.
Make the choice to let this go and be happy again. You are in control of your thoughts--they don't control you.