I'm affraid my girlfriend is gonna leave me because of my birth defect
Why do you think she will leave you because of your birth defect?
Just because I think my birth defect is ugly.
You shouldn't hide things from your partner. It's important for you to share so you can be completely honest with them.
But if I tell her she's going to want to leave me.
That's not necessarily true, I'm sure she wouldn't be so shallow to leave someone just because of a birth defect.