45 minutes is not a short time by any standards.
Nothing wrong with you at all in fact at 16 I could never last that long.
First off a lot of women just have a harder time cumming it's not your fault
second the longer you stay together the more you'll learn what turns her on and the easier it'll be for her to orgasm.
I suggest giving her more oral in time you can perfect that. Most women get really turned on by this and cum easier from oral than from sex. Even better get her wet with a little licking then medical question her.
Otherwise just don't worry bout it like I said lots of girls just do not cum as often while others seem to cum all the time it's just physiological differences nothing more nothing less.