here's what happens. when you are too anxious, your blood flow gets cut off from your body parts where you don't need it.
you go into flight or fight mode and your body pumps blood in your muscles that you use for defense and your brain. that's why for some people, their face turns red when you are nervous or anxious.
if you drink caffeine or take ephedrine or pills that 'awaken you' then it will do worse. i've actually had a panick attack about 2 years ago when i was with a girl. i was nervous because she had been the first girl in a long time. on top of that i had 2 large starbucks coffees + 2 caffeine pills and it was enough to give me a panick attack. i almost ended up in the hospital.
now my performance anxiety is gone except very rarely when i'm very excited about being with a new girl who i find extremely attractive or whatever. and that's totally fine, it's okay to be excited to be with an attractive woman.
but now it doesn't stop me from performing. maybe it slows me down a little bit and i need to MENTALLY relax myself first before i can do something.
if you are having performance anxiety and you're young (so theres no PHYSICAL problem with your penis) then just have 2-3 glasses of wine before sex. it will take the edge off the anxiety and believe me, you will have no problem to get hard as hell.
for young guys, two glasses of wine works better than viagra