Ok, i started masturbating, and had first sex at same age as you did(few years ago, I'm a little older). When i have sex, i cum during the first minute. But I don't loose erection after that. And cuming the second time may be realy hard for me, so I can understand your problem.
I can give you some advices that work for me. First of all you can try to make your gilfriend feel good, to understand what she wants, and give it to her. When i see that my girl really likes having sex with me, it feels good for me to.
Also try to experiment with poses. When you are on the top face to face ask her to put her legs together, this worked the best for me. Poses with deep penetration are good too. It feels really good to feel your dick touchg her (uterus/womb??). My gf has got a great body, so it's nice if i can see her boobs, tammy during sex, it also helps a little.
If you are from behind, rub her clit with same moves, that you do while masturbating. And one more thing, I had a gf that kissed my ear, and it was a big surprise for me that it's pleasant. Before that i thoght that only girls like such things, and guys have only one erogenic zone. Maybe you can find something like that for yourself (together with your gf, of course).
P.S. Sorry for my english, it's not my native language
