this doesn't sound like it's going to end well. very typical first-relationship stuff, actually- i went through quite a bit of that myself...
good sex can be found elsewhere. if you're really so big, there are plenty of other women who will appreciate that. if not and she's lying to make you feel better, well, there's plenty of women out there who will do that, too.
just know that you were right in the OP, you WILL regret not having more experience, and possibly even come to resent her more. best to get a start on this when you're still young.