Anxiety over "Average" penis size triggered again :/?

So basically, Im 20 years old and over the past year and half I've had on and off anxiety over my super average sized penis (5.5 inch fully erect measured from skin at base and about 4.5/4.6 around) average is about 5.1 inches measured same way apparently and so on, so I know I shouldn't really have anything to worry about.

However, last year I got super depressed with my first girlfriend over this. Ive taken my mind off it since and sometimes i don't worry at it all and laugh at how silly I am for worrying so much. However recently, Me and this super attractive girl have got to like each other and we kinda arranged to have sex next week haha. She then asked me jokingly if I have anything "big" and my heart kinda sunk and i played it cool saying im average and have nothing to brag about. She still wants my D though but since then, The anxiety has kicked in again and Im afraid it will prevent me from going ahead or that I will struggle to get it up on the day like with my first girlfriend. She will be my 2nd and first time in like over a year. How do i overcome all this? don't want to sound big headed, but for the sake of the argument, Im considered a pretty good looking guy, well built and attract stunners such as this girl and some have obviously had a few boyfriends who were probably bigger. However, this is the greatest insecurity of mine and it has prevented me from enjoying my late teens and now probably my twenties.

Has anyone else got this issue and overcome it?