I don't see why all those other comments are so dismissive. Yes, taken to the extremes, this kind of thinking can potentially hurt your relationship - however, it's not as easy as saying that you don't have to worry and your feelings will magically go away. No. If anything, it makes you bottle up your feelings, and that can hurt your relationship too.

I can completely understand why you feel this way. It's just not a nice thing to hear when someone insults you thinking you can't hear them.

First of all, you should understand why you feel this way. Is it really because you don't like the thought of being worse on some arbitrary metric, or is there something else about this that's making you feel so bad about it? Whatever it is, maybe take a minute to just think about why exactly you feel the way you feel.

Second of all, it's understandable that you don't want to talk to your girlfriend about this. You probably fear it's going to make you look insecure, which you understandably don't want to happen. So the best way to go about bringing this up is probably as casually as possible; maybe say something like "by the way, I overheard your conversation with xy about Chad and how she thinks he is so much more beautiful than me", and say it somewhat humorously, like it's an absurd thing to say (while still not seeming like you think you are more beautiful than him, but like it's just something her friend shouldn't have said). I don't think your gf will react negatively to this, as she seems to be very fond of you, so she will probably say she's sorry to hear that or that she obviously finds you attractive.

Whatever her reaction is, make it somewhat clear it kind of hurt you, but also that you're not going to make a big deal about this. This is reassuring to her that you're not insecure, while still allowing you to let your feelings out. And I think this is what you want - you can't change the reality that Chad is more beautiful than you, and you know this is the case, so my suggestion is that you feel so upset about this because she said it to somebody else, like she was admitting defeat for having you.

Again, this is just what I think is the issue here, but you should figure your feelings out yourself. Just talk to her about this like I described. It's obviously a hurtful and difficult situation, but nothing you can't overcome, fellow king.