Penis size can be the most important thing in the world to a man, but only because of what the female thinks. I have never known a man who cared about penis size for himself; he only pees with it a few times per day. Any comment about penis size to an average or less than average guy is nucleat in power and will always be taken as negative.
Penis size is unique. Unlike most all other problems, there is nothing that can be done to increase penis size. If you are too fat, lose weight. If you want different eye color get contacts. If your vagina is too loose, minor surgery will fix it. Boobs too small? A few thousand dollars will fix it.
But, if a man's penis is small he is the world's biggest loser and nothing but a punchline. However, if the female is happy with the man's penis, all is pretty much okay. If the female is perceived as the least bit unhappy with his penis size, the relationship is over.
And why not? If she is unhappy there is no question that he is therefore miserable because of it. He doesn't blame her, she didn't do anything wrong -- but neither did he. So, why be miserable. She will be happier with a larger guy and he will be happier with a girl whom he can satisfy.
I don't know what working through it means. Forcing a guy to watch you use large sex toys is excruciating and humiliating. Add to that, he will forever assume the girl is cheating on him. The only way to work through it, is to accept things the way they are and get out while you are both still young.
Saying, "Oh our relationship is okay except for his little penis," is like Mrs. Lincoln saying, "Oh the play was fine except for that."