New boyfriend excites my ex more than I did

I’m 18 and my girlfriend dumped me recently. I was ready for it to end, but my ex says that her new boyfriend excites her much more than I ever did. 

I’ve lost my confidence and, if I do have sex, my mind wanders and I worry that my penis is too small.

There is nothing wrong with you. You have just been in the wrong relationship. Your lack of sex drive is not permanent.