Errection problems?

look ... admittadly, in some peoples eyes, i'm probably abit too young to be thinking about at 16 ... but regardless of that it is the age of consent in my country and i need some help. It's happened to me a few times where i've wanted to have with either someone at a party ... or the girl i was going out with nearly a year ... admittadly she pestered me constantly about which i guess did put me off her ... but anyway basically ... i'd get all ready to go but then i wouldn't be able to get an errection. i thought i must have some kind of problem for a while but i realised i had no problem getting an errection while masturbating with or without any 'material'() but again i couldnt perform. i need some help i'm getting terrified of the IDEA of someoner making a p at me now ...