YOU ARE THE ADVICE COLUMNIST: How Do I Feel Secure "Down There" For My Girlfriend?

I'm insecure and have "short guy syndrome", help!
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So maybe this is just my ego talking but I feel like my girlfriend would be happier if I were bigger. I consider myself a slightly above average guy at around 8" and I feel like I know how to use it because she claims (I use the word claim because I think she does and she tells me she does but I honestly don't know how to tell if she were lying to me) to orgasm 10+ in one night of love making and I always put her first and ask her to be brutally honest me with what she wants and to honestly tell me if she doesn't get off because she's my main priority in bed.
However, one day (before her and I had ever had sex), my girlfriend and I were on the topic of some of her exes (she chose this topic of conversation) and she mentioned one guy who blew her mind and made love to her eight times in one night. While I would love to do that with her, eight times is kind of impossible because it literally takes me sometimes upwards of four hours to get mine because even though I could cum and take a break and keep going, I would rather just not take any breaks at all and just go all night. I asked a little bit about him and she just blew it off like nothing and tried to ease my mind by saying that he was quite small and that I shouldn't worry about it. He came up in conversation a few weeks later and she mentioned something about him again but this time it was about his size and that he was quite well endowed. I prodded a little bit for more information before she finally told me that he was 12.5" and now I can't help but think about her having sex with this random guy who I don't even know. 
It also doesn't help that on several occasions, she's mentioned other guys who she (although has never had sex with them) claims that they, in her words, "have horse dicks."  I've asked her if she would enjoy it more if I were bigger and she says that she wouldn't but I can't help but be a little concerned.  Am I right to worry? Or am I just being an average guy with "short guy syndrome"? 
Do you have any advice for an insecure body image for this man? Let him know in the comments. 
 
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