Well firstly she owes you more respect.

Brag about your exes having tighter cunts/nicer tits/better lips and you would be out on your ass. Your cock is way big enough.
YouTube - Curb Your Enthusiasm - Big Vagina
You have two suggestable options in imho..
1) get a biiiig dildo and fuck her silly while she blows you. Ruin her - she sounds like she would enjoy it. Make sure to film it and put it on RedTube
2) dump her
Personally, I would do both and do it in that order too.
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On a serious note, if she does value you, you should be able to talk to her and tell her how this makes you feel. If you can overcome your hangup, you may even be able to buy into the fantasy too...