i think you are getting stressed for a nothing....and i also think you are slowly getting into depression so active up and live life...we are so young!!
The size is the same......it is just your brain that are tricking you!! We all have that problem once in a time!!! And no one makes a mistake while masturbating unless they break the penis!! Who never retain the ejaculation??? At least me and some of my friends did it or still do it!! You have no problems....you are just stressing for a nothing!! And if your gf does not like your penis and leaves you then she not worth you coz she will be just a slut--sorry--
hope i helped