without my asking that 3 of the 5 other men she's been with had penises much bigger than mine.

Why would she do that? And does she really think she can reconstruct her exes penis size exactly? Lots of people have actually an idea how long/big something actually is.

And worse, when I told my girlfriend about this insecurity, she only made me feel worse.

Why would you feel low about yourself? Fuck this, your gf is a tool. Have you actually called her out on it? How are her comments productive for this relationship? How would she feel if you told her that all other girls had tighter vaginas? Or bigger/perkier boobs?

At a little over 7 inches,

That's quite big already.....