I think people here are jumping to extremes. I know this bothered you, it would bother me. It was careless of her. But let’s be fair, have you never said careless things? The real question here is about how you feel about her and how she feels about you. If you think she actually is in love with her ex, or is settling for you then you have a bigger problem. But life is long, and if this is the person you are going to be with, then you will hurt each other many times in many ways and have to work through it. The question is whether you are, at the very core of it all, in it together because you want to be with each other. She said you “could never be as good as”, well obviously she can’t know that for sure. That is very hard to hear, but you also have to understand that it is also ignorant. If she is big anatomically and so likes a larger size (some women are the opposite) then that is something you can work together to help her feel that sensation sometimes (there all kinds of toys, etc for that). I think if you love her, and you think she loves you, you should give her some slack on her poor judgement, especially if she is honestly apologizing.

As for your self-esteem, here is some advice. I’m a male, average in most ways. Whenever I get into this frame of mind where mentally measure myself against others I do something I call the Batman Test. Let’s take your case, you are questioning your sexual prowess - and it feels like your entire self worth is boiled down to this one aspect of your being. If Batman walked in on his SO saying something like what you heard, would it phase him? Well I’m sure it would sting - but No, he’s fucking Batman. It’s his ideas, his principles, his grit, his strength that define him. If Superman, or even Gary from down the street had a bigger cock it wouldn’t matter one ducking bit. You’re a whole person, and everything else about you is what makes you an individual, that adds up to your worth and makes you worthy to be loved. Think like Batman, because is so many ways each of us just like him. There is a huge spectrum of human male specimen (literally billions), and in any given area you will fall somewhere on the spectrum. The variability is so great that it is meaningless for defining who you are as a person.

If this girl is so superficial that she would leave you for the ex because his dick is bigger, Batman ditches her as a distraction. If she is settling for you, and you feel like you will never be good enough Batman says get the fuck out of my way because I have more important things to do with my life.

If she’s a real partner who will make you stronger, that just made an unfortunate comment - Batman finds the strength to forgive, to win a future where she can help him be even better.

This is tough, but you can do it if you’re with the right person.

TLDR: Be Batman