I can achieve full erection when masturbating, but when I was with a girl I could not get it up

So for the first time recently i got a girl in bed and i could not get it up, i was embarrassed and confused as of why and looked on the internet for help and found nothing, when i watch porn i can get it up but they don't excite me as much, i still get it up but not all the way i feel like im a little far away from getting a full erection but just cant hit it. so the main problem is i cant get a full erection and when i tried to have sex with that girl i couldn't event get it up, im 19 and not sure what to do. please help me out,