i have this problem too, although i am 18 years old and i should be fully functioning.
i believe it is any distraction, if you use a condom, taking time to open it and put it on your penis can distract you and it can go down. you could try some P.C muscle exercises, this is the muscle that controlls your bladder, you can feel it between your anus and your penis, try to tense your penis and you can feel it contracting. find p.c muscle exercises online! it has started to work for me so i hope it works for you! :)
Maybe it happened the first time for some unrelated reason. After that you became afraid of it happening again so you're mentally causing it, expecting it to happen so you're actually making it happen.
Maybe you're affraid of her becoming pregnant....