I wish I had a magic answer for you. It looks like this is more of a self-esteem issue than anything else. If you doubt her when she says that loves your cock, then you may have some trust issues in there too, but since you seemed to doubt yourself more than her that may not be so much of an issue, which is good. Building a little self-esteem is much easier than building trust.
There are many ways to try and boost yourself a little. First, looking good is a nice first step. I have no idea what body type you have, but a simple way to boost your self image is to hit the gym a little more often. You will feel better and stronger and I have found that can lead to greater sexual confidence.
Second, don't let sex revolve around just your cock. Granted, it is an integral part of sex, but still just one part. Obviously oral sex comes to mind, but let me give you a personal example. Every week my wife and I shower together. I caress, kiss, and lick EVERY inch of her body, and I do it slowly, while spending remarkably little time with her pussy. I take that as a time to make sure that she knows that I desire every inch of her, not just her sexual organs and tits. While doing so, I make sure I make sure I rub my erection against her thighs a bit to let her know how much I desire her. In doing this, I can usually, though not always, bring her to an orgasm, every once in a while, even two, with very little, if any, attention to her clit. I get off on it almost as much as she does. It gives me greater sexual self confidence than actual intercourse and it has nothing to do with my cock, let alone the size of it.
Lastly, I don't think there is really anything wrong in occasionally desiring a bigger cock. We all have our self-doubts of one kind or another. Just don't let it negatively impact your life, especially when your girlfriend seems to prize it as much as she does.